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: NMaxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinctrl-st.c

@@ -460,6 +460,20 @@ static void st_pctl_set_function(struct st_pio_control *pc,
 	regmap_field_write(alt, val);
 }
 
+static unsigned int st_pctl_get_pin_function(struct st_pio_control *pc, int pin)
+{
+	struct regmap_field *alt = pc->alt;
+	unsigned int val;
+	int offset = pin * 4;
+
+	if (!alt)
+		return 0;
+
+	regmap_field_read(alt, &val);
+
+	return (val >> offset) & 0xf;
+}
+
 static unsigned long st_pinconf_delay_to_bit(unsigned int delay,
 	const struct st_pctl_data *data, unsigned long config)
 {