pinctrl: meson-axg: Introduce a pinctrl pinmux ops for Meson-AXG SoC

The pin controller has been updated in the Amlogic Meson AXG series,
which use continuous 4-bit register to select function for each pin.
In order to support this, a new pinmux operations "meson_axg_pmx_ops"
has been added.

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Xingyu Chen <xingyu.chen@amlogic.com>
Signed-off-by: Yixun Lan <yixun.lan@amlogic.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>