ARM: dts: imx6qdl-sabrelite: use GPIO_6 for FEC interrupt.

This works around a hardware bug.
From "Chip Errata for the i.MX 6Dual/6Quad"

ERR006687 ENET: Only the ENET wake-up interrupt request can wake the
system from Wait mode.

The ENET block generates many interrupts. Only one of these interrupt lines
is connected to the General Power Controller (GPC) block, but a logical OR
of all of the ENET interrupts is connected to the General Interrupt Controller
(GIC). When the system enters Wait mode, a normal RX Done or TX Done does not
wake up the system because the GPC cannot see this interrupt. This impacts
performance of the ENET block because its interrupts are serviced only when
the chip exits Wait mode due to an interrupt from some other wake-up source.

Before this patch, ping times of a Sabre Lite board are quite
random:
ping 192.168.0.13 -i.5 -c5
PING 192.168.0.13 (192.168.0.13) 56(84) bytes of data.
64 bytes from 192.168.0.13: icmp_req=1 ttl=64 time=15.7 ms
64 bytes from 192.168.0.13: icmp_req=2 ttl=64 time=14.4 ms
64 bytes from 192.168.0.13: icmp_req=3 ttl=64 time=13.4 ms
64 bytes from 192.168.0.13: icmp_req=4 ttl=64 time=12.4 ms
64 bytes from 192.168.0.13: icmp_req=5 ttl=64 time=11.4 ms

=== 192.168.0.13 ping statistics ===
5 packets transmitted, 5 received, 0% packet loss, time 2004ms
rtt min/avg/max/mdev = 11.431/13.501/15.746/1.508 ms
____________________________________________________
After this patch:

ping 192.168.0.13 -i.5 -c5
PING 192.168.0.13 (192.168.0.13) 56(84) bytes of data.
64 bytes from 192.168.0.13: icmp_req=1 ttl=64 time=0.120 ms
64 bytes from 192.168.0.13: icmp_req=2 ttl=64 time=0.175 ms
64 bytes from 192.168.0.13: icmp_req=3 ttl=64 time=0.169 ms
64 bytes from 192.168.0.13: icmp_req=4 ttl=64 time=0.168 ms
64 bytes from 192.168.0.13: icmp_req=5 ttl=64 time=0.172 ms

=== 192.168.0.13 ping statistics ===
5 packets transmitted, 5 received, 0% packet loss, time 1999ms
rtt min/avg/max/mdev = 0.120/0.160/0.175/0.026 ms
____________________________________________________

Also, apply same change to imx6qdl-nitrogen6x.

This change may not be appropriate for all boards.
Sabre Lite uses GPIO6 as a power down output for a ov5642
camera. As this expansion board does not yet work with mainline,
this is not yet a conflict. It would be nice to have an alternative
fix for boards where this is a problem.

For example Sabre SD uses GPIO6 for I2C3_SDA. It also
has long ping times currently. But cannot use this fix
without giving up a touchscreen.

Its ping times are also random.

ping 192.168.0.19 -i.5 -c5
PING 192.168.0.19 (192.168.0.19) 56(84) bytes of data.
64 bytes from 192.168.0.19: icmp_req=1 ttl=64 time=16.0 ms
64 bytes from 192.168.0.19: icmp_req=2 ttl=64 time=15.4 ms
64 bytes from 192.168.0.19: icmp_req=3 ttl=64 time=14.4 ms
64 bytes from 192.168.0.19: icmp_req=4 ttl=64 time=13.4 ms
64 bytes from 192.168.0.19: icmp_req=5 ttl=64 time=12.4 ms

=== 192.168.0.19 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 12.451/14.369/16.057/1.316 ms

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
CC: Ranjani Vaidyanathan <ra5478@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>