ARM: dts: ls1021a: Rename 'mdio-mux-emi1' to 'mdio-mux@54'

ARM: dts: ls1021a: Rename 'mdio-mux-emi1' to 'mdio-mux@54'

Rename 'mdio-mux-emi1' to 'mdio-mux@54'.

Add fallback compatible string mdio-mux.

Fix below warning:
  arch/arm/boot/dts/nxp/ls/ls1021a-qds.dtb: memory-controller@1530000 (fsl,ifc): board-control@3,0: 'oneOf' conditional failed, one must be fixed:
        'bank-width', 'device-width', 'mdio-mux-emi1' do not match any of the regexes: '^mdio-mux@[a-f0-9,]+$', 'pinctrl-[0-9]+'

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>