mmc: dt-bindings: update bindings doc to support Meson-AXG SoC

Update the documentation to list support for Meson-AXG SoC explicitly.
The new binding string is necessary since this SoC introduce a few
IP difference comparing to previous old generation.
Signed-off-by: Nan Li <nan.li@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Kevin Hilman <khilman@baylibre.com>

Documentation/devicetree/bindings/mmc/amlogic,meson-gx.txt

@@ -12,6 +12,7 @@ Required properties:
   - "amlogic,meson-gxbb-mmc"
   - "amlogic,meson-gxl-mmc"
   - "amlogic,meson-gxm-mmc"
+  - "amlogic,meson-axg-mmc"
 - clocks     : A list of phandle + clock-specifier pairs for the clocks listed in clock-names.
 - clock-names: Should contain the following:
 	"core" - Main peripheral bus clock