ARM: dts: keystone: Fix the mdio bindings by moving it to soc specific file

Currently mdio bindings are defined in keystone.dtsi and this results
in incorrect unit address for the node on K2E and K2L SoCs. Fix this
by moving them to SoC specific DTS file.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>