pinctrl: stm32: Handle RSVD pin configuration

When a pin is assigned to a coprocessor not running Linux, the pin
should be flagged in the DT as reserved and not usable by Linux.

Add pin configuration RSVD (Reserved) and instrument the driver to
not update the mux setting.

Signed-off-by: Fabien Dessenne <[e-mail]>
Signed-off-by: Antonio Borneo <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

drivers/pinctrl/stm32/pinctrl-stm32.c

···
 	"af8", "af9", "af10",
 	"af11", "af12", "af13",
 	"af14", "af15", "analog",
+	"reserved",
 };
 
 struct stm32_pinctrl_group {
···
 		if (pin->pin.number != pin_num)
 			continue;
 
+		if (fnum == STM32_PIN_RSVD)
+			return true;
+
 		for (k = 0; k < STM32_CONFIG_NUM; k++) {
 			if (func->num == fnum)
 				return true;
···
 	if (!range) {
 		dev_err(pctl->dev, "No gpio range defined.\n");
 		return -EINVAL;
+	}
+
+	if (function == STM32_PIN_RSVD) {
+		dev_dbg(pctl->dev, "Reserved pins, skipping HW update.\n");
+		return 0;
 	}
 
 	bank = gpiochip_get_data(range->gc);

drivers/pinctrl/stm32/pinctrl-stm32.h

···
 #define STM32_PIN_GPIO		0
 #define STM32_PIN_AF(x)		((x) + 1)
 #define STM32_PIN_ANALOG	(STM32_PIN_AF(15) + 1)
-#define STM32_CONFIG_NUM	(STM32_PIN_ANALOG + 1)
+#define STM32_PIN_RSVD		(STM32_PIN_ANALOG + 1)
+#define STM32_CONFIG_NUM	(STM32_PIN_RSVD + 1)
 
 /*  package information */
 #define STM32MP_PKG_AA		BIT(0)