arm64: a37xx: pinctrl: Implement get_pins_count, get_pin_name and get_pin_muxing functions

These functions are required for 'pinmux status -a' command to print
current configuration of each MPP pin.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>