As we have support for this in board-rx51-peripherals.c, let's
add it to the .dts files too.

Note that the reset GPIO will eventually go to the driver.
For now let's just pull it down and skip any further reset
in case the bootloader has configured the MAC address so
NFSroot works.

Also note that after 3430-sdp are using proper GPMC timings
we can remove the tests for smsc,lan91c94 in gpmc.c.
Signed-off-by: NTony Lindgren <tony@atomide.com>