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

Update the doc to explicitly add Meson-AXG to support list

Signed-off-by: Jian Hu <jian.hu@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>