arm64: dts: zii-ultra: update MDIO speed and preamble

Update MDIO configuration with zii-ultra device to fully utilize
MDIO endpoint capabilities.  Device supports 12.5MHz clock and
doesn't require MDIO preamble.

Signed-off-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>