ARM: dts: ZII: update MDIO speed and preamble

Update MDIO configuration with ZII devices to fully utilize
MDIO endpoint capabilities.  All devices support 12.5MHz clock and
don't require MDIO preable.

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