ARM: dts: meson-sm1: add U-Boot specific DT for graphics

Rename meson-g12a-u-boot.dtsi into meson-g12-common-u-boot.dtsi to
match the new DT architecture and add meson-sm1-sei610-u-boot.dtsi
to handle the U-Boot specific DT for graphics.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>