pinctrl: meson-gx: add support for getting pinmux status

In order to support the "pinmux status" command, use the common functions
to get the pins count and names, and add the GX specific function to get
the current function from registers.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Maxime Jourdan <mjourdan@baylibre.com>