imx8mm-evk has a GPIO connected to AR8031 Ethernet PHY's reset pin.

Describe it in the device tree, following phy's datasheet reset duration of 10ms.

Tested booting via NFS.
Signed-off-by: Alifer Moraes <alifer.wsdm@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>