arm64: dts: move common G12A & G12B modes to meson-g12-common.dtsi

To simplify the representation of differences betweem the G12A and G12B
SoCs, move the common nodes into a meson-g12-common.dtsi file and
express the CPU nodes and differences in meson-g12a.dtsi and meson-g12b.dtsi.

This separation will help for DVFS and future Amlogic SM1 Family support.

The sd_emmc_a quirk is added in the g12a/g12b since since it's already
known the sd_emmc_a controller is fixed in the next SM1 SoC family.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>