arm64: dts: meson-axg: fix MHU compatible

This fixes the following DT schemas check errors:
meson-axg-s400.dt.yaml: mailbox@ff63c404: 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>