arm64: dts: imx8mp: Add DT overlay describing i.MX8MP DHCOM SoM rev.100

The current imx8mp-dhcom-som.dtsi describes production rev.200 SoM,
add DT overlay which reinstates rev.100 SoM description to permit
prototype rev.100 SoMs to be used until they get phased out.

Signed-off-by: Marek Vasut <marex@denx.de>