ACPI / OSI: Add OEM _OSI string to enable NVidia HDMI audio

Some ThinkPad systems have a power-saving feature that turns off HDMI
audio device in Windows, but NVidia Linux driver does not support this
feature. As a result, HDMI audio will not work on Linux.

A BIOS workaround is added with an OEM_OSI string
"Linux-Lenovo-NV-HDMI-Audio" to power on NVidia HDMI audio when booting.

The form of the OEM _OSI strings is defined by each OEMs and is
discussed in Documentation/acpi/osi.txt.
Signed-off-by: Alex Hung <alex.hung@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/osi.c

@@ -66,6 +66,14 @@ osi_setup_entries[OSI_STRING_ENTRIES_MAX] __initdata = {
 	 * be removed if both new and old graphics cards are supported.
 	 */
 	{"Linux-Dell-Video", true},
+	/*
+	 * Linux-Lenovo-NV-HDMI-Audio is used by BIOS to power on NVidia's HDMI
+	 * audio device which is turned off for power-saving in Windows OS.
+	 * This power management feature observed on some Lenovo Thinkpad
+	 * systems which will not be able to output audio via HDMI without
+	 * a BIOS workaround.
+	 */
+	{"Linux-Lenovo-NV-HDMI-Audio", true},
 };
 
 static u32 acpi_osi_handler(acpi_string interface, u32 supported)