net: fec: set GPR bit on suspend by DT configuration.

On some SoCs, such as the i.MX6, it is necessary to set a bit
in the SoC level GPR register before suspending for wake on lan
to work.

The fec platform callback sleep_mode_enable was intended to allow this
but the platform implementation was NAK'd back in 2015 [1]

This means that, currently, wake on lan is broken on mainline for
the i.MX6 at least.

So implement the required bit setting in the fec driver by itself
by adding a new optional DT property indicating the GPR register
and adding the offset and bit information to the driver.

[1] https://www.spinics.net/lists/netdev/msg310922.html



Signed-off-by: Martin Fuzzey <martin.fuzzey@flowbird.group>
Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>