net: macb: Fix ZynqMP SGMII non-wakeup source resume failure

When GEM is in SGMII mode and disabled as a wakeup source, the power
management controller can power down the entire full power domain(FPD)
if none of the FPD devices are in use.

Incase of FPD off, there are below ethernet link up issues on non-wakeup
suspend/resume. To fix it add phy_exit() in suspend and phy_init() in the
resume path which reinitializes PS GTR SGMII lanes.

$ echo +20 > /sys/class/rtc/rtc0/wakealarm
$ echo mem > /sys/power/state

After resume:

$ ifconfig eth0 up
xilinx-psgtr fd400000.phy: lane 0 (type 10, protocol 5): PLL lock timeout
phy phy-fd400000.phy.0: phy poweron failed --> -110
xilinx-psgtr fd400000.phy: lane 0 (type 10, protocol 5): PLL lock timeout
SIOCSIFFLAGS: Connection timed out
phy phy-fd400000.phy.0: phy poweron failed --> -110

Fixes: 8b73fa3a

 ("net: macb: Added ZynqMP-specific initialization")
Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>