ARM: dts: meson: add the hardware random number generator

All supported Meson SoCs have a random number generator in CBUS.
Newer SoCs (GXBB, GXL and GXM) provide only one 32-bit random number
register, whereas the older SoCs (Meson6, Meson8 and Meson8b) have two
32-bit random number registers. The existing meson-rng driver only
supports the lower 32-bit - but it still works fine on the older SoCs
apart from this small limitation.

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