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>

arch/arm64/boot/dts/amlogic/meson-axg.dtsi

@@ -1118,7 +1118,7 @@ clkc: clock-controller {
 		};
 
 		mailbox: mailbox@ff63c404 {
-			compatible = "amlogic,meson-gx-mhu", "amlogic,meson-gxbb-mhu";
+			compatible = "amlogic,meson-gxbb-mhu";
 			reg = <0 0xff63c404 0 0x4c>;
 			interrupts = <GIC_SPI 208 IRQ_TYPE_EDGE_RISING>,
 				     <GIC_SPI 209 IRQ_TYPE_EDGE_RISING>,