pinctrl: update GPIO range doc

This updates the GPIO range documentation with the API changes
for sparse/random/arbitrary pin-to-GPIO mappings.
Reviewed-by: Christian Ruppert <christian.ruppert@abilis.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/pinctrl.txt

@@ -350,6 +350,23 @@ chip b:
  - GPIO range : [48 .. 55]
  - pin range  : [64 .. 71]
 
+The above examples assume the mapping between the GPIOs and pins is
+linear. If the mapping is sparse or haphazard, an array of arbitrary pin
+numbers can be encoded in the range like this:
+
+static const unsigned range_pins[] = { 14, 1, 22, 17, 10, 8, 6, 2 };
+
+static struct pinctrl_gpio_range gpio_range = {
+	.name = "chip",
+	.id = 0,
+	.base = 32,
+	.pins = &range_pins,
+	.npins = ARRAY_SIZE(range_pins),
+	.gc = &chip;
+};
+
+In this case the pin_base property will be ignored.
+
 When GPIO-specific functions in the pin control subsystem are called, these
 ranges will be used to look up the appropriate pin controller by inspecting
 and matching the pin to the pin ranges across all controllers. When a
@@ -357,9 +374,9 @@ pin controller handling the matching range is found, GPIO-specific functions
 will be called on that specific pin controller.
 
 For all functionalities dealing with pin biasing, pin muxing etc, the pin
-controller subsystem will subtract the range's .base offset from the passed
-in gpio number, and add the ranges's .pin_base offset to retrive a pin number.
-After that, the subsystem passes it on to the pin control driver, so the driver
+controller subsystem will look up the corresponding pin number from the passed
+in gpio number, and use the range's internals to retrive a pin number. After
+that, the subsystem passes it on to the pin control driver, so the driver
 will get an pin number into its handled number range. Further it is also passed
 the range ID value, so that the pin controller knows which range it should
 deal with.
@@ -368,6 +385,7 @@ Calling pinctrl_add_gpio_range from pinctrl driver is DEPRECATED. Please see
 section 2.1 of Documentation/devicetree/bindings/gpio/gpio.txt on how to bind
 pinctrl and gpio drivers.
 
+
 PINMUX interfaces
 =================