arm64: dts: add *_skew-ps properties for micrel phy node

Because of the commit bcf3440c ("net: phy: micrel: add phy-mode
support for the KSZ9031 PHY"), the function ksz9031_config_rgmii_delay
is introduced to initialize the *_PAD_SKEW registers. But these initial
values are not suitable for axm56xx board. So read the workable value
before ksz9031_config_rgmii_delay and then rewrite them to registers
again.

So use *-skew-ps device tree properties in device tree files to
reinitialise *_PAD_SKEW registers.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>