pinctrl: single: add get_pin_muxing operation

It allows to display the muxing of a given pin. Inspired by more recent
versions of the Linux driver, in addition to the address and the value
of the configuration register I added the pin function retrieved from
the DT. In doing so, the information displayed does not depend on the
platform, being a generic type driver, and it can be useful for debug
purposes.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Simon Glass <sjg@chromium.org>