ARM64: dts: Add support for Meson GXM

Following the Amlogic Linux kernel, it seem the only differences
between the GXL and GXM SoCs are the CPU Clusters.

This commit renames the gxl-s905d-p23x DTSI in a common file for
S905D p23x and S912 q20x boards.

Then adds a meson-gxm dtsi and reproduce the P23x to Q20x boards
dts files since the S905D and S912 SoCs shares the same pinout
and the P23x and Q20x boards are identical.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>