sunxi: pinctrl: Implement get_pin_muxing function

The pinmux command uses this function to display pinmux status.

Since the driver cannot map pin numbers to a list of supported
functions, only functions which are common across all pins can be
reported by name.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>