arm64: dts: meson-gx: fix watchdog compatible

This fixes the following DT schemas check errors:
meson-gxbb-nanopi-k2.dt.yaml: watchdog@98d0: compatible:0: 'amlogic,meson-gx-wdt' is not one of ['amlogic,meson-gxbb-wdt']
meson-gxl-s805x-libretech-ac.dt.yaml: watchdog@98d0: compatible:0: 'amlogic,meson-gx-wdt' is not one of ['amlogic,meson-gxbb-wdt']

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