ACPI / property: Document RS485 _DSD properties

Document RS485 related properties returned from ACPI _DSD as device
properties.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220426122448.38997-5-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/firmware-guide/acpi/enumeration.rst

@@ -390,6 +390,31 @@ descriptors once the device is released.
 See Documentation/firmware-guide/acpi/gpio-properties.rst for more information
 about the _DSD binding related to GPIOs.
 
+RS-485 support
+==============
+
+ACPI _DSD (Device Specific Data) can be used to describe RS-485 capability
+of UART.
+
+For example::
+
+	Device (DEV)
+	{
+		...
+
+		// ACPI 5.1 _DSD used for RS-485 capabilities
+		Name (_DSD, Package ()
+		{
+			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
+			Package ()
+			{
+				Package () {"rs485-rts-active-low", Zero},
+				Package () {"rs485-rx-active-high", Zero},
+				Package () {"rs485-rx-during-tx", Zero},
+			}
+		})
+		...
+
 MFD devices
 ===========