ARM64: dts: meson-gxm: add SCPI configuration for GXM

This adds the SCPI DVFS clock index and configures the CPU cores
accordingly.

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