mux: gpio: add optional regulator support

Some of the external muxes needs powering up using a regulator.
This is the case with Lenovo T14s laptop which has a external audio mux
to handle US/EURO headsets.

Add support to the driver to handle this optional regulator.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Tested-by: Christopher Obbard <christopher.obbard@linaro.org>
Reviewed-by: Johan Hovold <johan+linaro@kernel.org>
Tested-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20250327100633.11530-3-srinivas.kandagatla@linaro.org
[krzk: Adjust dev_err message per Johan's review]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>

Author: Srinivas-Kandagatla

drivers/mux/gpio.c

@@ -15,6 +15,7 @@
 #include <linux/mux/driver.h>
 #include <linux/platform_device.h>
 #include <linux/property.h>
+#include <linux/regulator/consumer.h>
 
 struct mux_gpio {
 	struct gpio_descs *gpios;
@@ -80,6 +81,10 @@ static int mux_gpio_probe(struct platform_device *pdev)
 		mux_chip->mux->idle_state = idle_state;
 	}
 
+	ret = devm_regulator_get_enable_optional(dev, "mux");
+	if (ret && ret != -ENODEV)
+		return dev_err_probe(dev, ret, "failed to get/enable mux supply\n");
+
 	ret = devm_mux_chip_register(dev, mux_chip);
 	if (ret < 0)
 		return ret;