dm: pinctrl: Add pinctrl_get_pin_name and pinctrl_get_pins_count

Add pinctrl_get_pin_name() and pinctrl_get_pins_count() methods
to obtain pin's name and pin's muxing given a pin reference.

This will be used by the new pinmux command.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>