arm64: dts: meson-gx: fix mhu compatible

This fixes the following DT schemas check errors:
meson-gxbb-nanopi-k2.dt.yaml: mailbox@404: compatible:0: 'amlogic,meson-gx-mhu' is not one of ['amlogic,meson-gxbb-mhu']
meson-gxl-s805x-libretech-ac.dt.yaml: mailbox@404: compatible:0: 'amlogic,meson-gx-mhu' is not one of ['amlogic,meson-gxbb-mhu']

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