dt-bindings: i2c: update documentation for the Meson-AXG

Update the doc to explicitly add Meson-AXG to support list
Signed-off-by: NJian Hu <jian.hu@amlogic.com>
Signed-off-by: NYixun Lan <yixun.lan@amlogic.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c-meson.txt

 Amlogic Meson I2C controller
 
 Required properties:
- - compatible: must be "amlogic,meson6-i2c" or "amlogic,meson-gxbb-i2c"
+ - compatible: must be:
+	"amlogic,meson6-i2c" for Meson8 and compatible SoCs
+	"amlogic,meson-gxbb-i2c" for GXBB and compatible SoCs
+	"amlogic,meson-axg-i2c"for AXG and compatible SoCs
+
  - reg: physical address and length of the device registers
  - interrupts: a single interrupt specifier
  - clocks: clock for the device