Documentation: gpio: Update ACPI part of the document to mention _DSD

With ACPI 5.1 _DSD (Device Specific Data) it is now possible to name
functions just like Device Tree is doing. Make sure that the documentation
mentions _DSD as the recommended way to describe GPIOs in ACPI systems.

Reported-by: Darren Hart <[e-mail]>
Reviewed-by: Darren Hart <[e-mail]>
Signed-off-by: Mika Westerberg <[e-mail]>
Acked-by: Rafael J. Wysocki <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/gpio/board.txt

···
 
 ACPI
 ----
-ACPI does not support function names for GPIOs. Therefore, only the "idx"
-argument of gpiod_get_index() is useful to discriminate between GPIOs assigned
-to a device. The "con_id" argument can still be set for debugging purposes (it
-will appear under error messages as well as debug and sysfs nodes).
+ACPI also supports function names for GPIOs in a similar fashion to DT.
+The above DT example can be converted to an equivalent ACPI description
+with the help of _DSD (Device Specific Data), introduced in ACPI 5.1:
+
+	Device (FOO) {
+		Name (_CRS, ResourceTemplate () {
+			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
+				"\\_SB.GPI0") {15} // red
+			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
+				"\\_SB.GPI0") {16} // green
+			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
+				"\\_SB.GPI0") {17} // blue
+			GpioIo (Exclusive, ..., IoRestrictionOutputOnly,
+				"\\_SB.GPI0") {1} // power
+		})
+
+		Name (_DSD, Package () {
+			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
+			Package () {
+				Package () {
+					"led-gpios",
+					Package () {
+						^FOO, 0, 0, 1,
+						^FOO, 1, 0, 1,
+						^FOO, 2, 0, 1,
+					}
+				},
+				Package () {
+					"power-gpios",
+					Package () {^FOO, 3, 0, 0},
+				},
+			}
+		})
+	}
+
+For more information about the ACPI GPIO bindings see
+Documentation/acpi/gpio-properties.txt.
 
 Platform Data
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