arm64: dts: lx2160a: specify clock frequencies for the MDIO controllers

Up until now, the external MDIO controller frequency values relied
either on the default ones out of reset or on those setup by u-boot.
Let's just properly specify the MDC frequency in the DTS so that even
without u-boot's intervention Linux can drive the MDIO bus.

Fixes: 6e1b8fae ("arm64: dts: lx2160a: add emdio1 node")
Fixes: 5705b9dc ("arm64: dts: lx2160a: add emdio2 node")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>