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: 0420dde3 ("arm64: dts: ls208xa: add the external MDIO nodes")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>