Describe the GPIO used to reset the Ethernet PHY for EthernetAVB.
This allows the driver to reset the PHY during probe and after system
resume.

On ULCB, the enable pin of the regulator providing PHY power is always
pulled high, but the driver may still need to reset the PHY if this
wasn't done by the bootloader before.

Inspired by patches in the BSP for the individual Salvator-X/XS boards
by Kazuya Mizuguchi.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>