ARM: dts: am43xx-epos-evm: fix pinmux node layout

Pinmux node should be a reference to the base one, not a complete re-write
of it. Having the node like this also prevents modifying the node layout
in the base am4372.dtsi file, which is needed for control module changes.

Signed-off-by: Tero Kristo <t-kristo@ti.com>