arm64: dts: meson-g12b-ugoos-am6: fix board compatible

Add missing amlogic,s922x in the board compatible list.

It fixes:
meson-g12b-ugoos-am6.dt.yaml: /: compatible: ['ugoos,am6', 'amlogic,g12b'] is not valid under any of the given schemas

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200326165958.19274-4-narmstrong@baylibre.com