pinctrl: st: Introduce a 'get pin function' call

This call fetches the numerical function value a specified pin is
currently operating in.  Function zero is more often than not the
GPIO function.  Greater than zero values represent an alternative
function.  You'd need to either look those up in the Device Tree
sources or the Programmer's Manual.

Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>