documentation: Add compatibles for Amlogic Meson AXG pin controllers

Add compatibles for Amlogic Meson AXG pin controllers

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Rob Herring <robh at kernel.org>
Signed-off-by: Xingyu Chen <xingyu.chen@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>