arm64: dts: rockchip: px30: Move dmc into -u-boot.dtsi

dmc node is specific to U-Boot, it is always better practice
to maintain U-Boot specific nodes into -u-boot.dtsi files
in order to maintain Linux dts file sync compatibility.

Move the dmc into px30-u-boot.dtsi, also add dmc node
explicitly in rk3326-odroid-go2-u-boot.dtsi since it is
using px30.dts.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>