gpio: add GPIO_GET_PIN_LIST

While most GPIO controllers provide pins from 0 .. N in a sequential
manner, not all controllers start with pin 0, and not all controllers
order their pins sequentially. Allow callers to get a pin list from the
controller. The default behaviour is to fill pin_list with pins 0 to
GPIO_PIN_MAX().

Suggested by:	mmel
Reviewed by:	mmel
Approved by:	imp (mentor)
Differential Revision:	https://reviews.freebsd.org/D52172

Author: VexedUXR

sys/dev/gpio/gpio_if.m

@@ -62,6 +62,22 @@
 
 		return (0);
 	}
+
+	static int
+	gpio_default_get_pin_list(device_t dev, uint32_t *pin_list)
+	{
+		uint32_t maxpin;
+		int err;
+
+		err = GPIO_PIN_MAX(dev, &maxpin);
+		if (err != 0)
+			return (ENXIO);
+
+		for (int i = 0; i <= maxpin; i++)
+			pin_list[i] = i;
+
+		return (0);
+	}
 };
 
 HEADER {
@@ -185,3 +201,13 @@
 	uint32_t num_pins;
 	uint32_t *pin_flags;
 } DEFAULT gpio_default_nosupport;
+
+#
+# Get the controller's pin numbers. pin_list is expected to be an array with at
+# least GPIO_PIN_MAX() elements. Populates pin_list from 0 to GPIO_PIN_MAX() by
+# default.
+#
+METHOD int get_pin_list {
+	device_t dev;
+	uint32_t *pin_list;
+} DEFAULT gpio_default_get_pin_list;