arm64: dts: imx8mp-evk: Improve the Ethernet PHY description

According to the datasheet RTL8211, it must be asserted low for at least
10ms and at least 72ms "for internal circuits settling time" before
accessing the PHY registers.

Add properties to describe such requirements.

Reported-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>