drivers/gpio_keys: Add SW_MUTE_DEVICE

Added SW_MUTE_DEVICE event type for mic mute switch.

BUG=b:184593945
BRANCH=puff
TEST=build image and verify with evtest on puff:
/dev/input/event3:	mic_mute_switch
UI event_device_info receives the proper name.

Change-Id: I09c52dc3df63e266c73741b102a22f8a2b896791
Signed-off-by: Ben Zhang <benzh@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54689
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/drivers/generic/gpio_keys/chip.h

@@ -14,6 +14,7 @@ enum {
 
 /* Switch events type (Linux code emitted for EV_SW) */
 enum {
+	SW_MUTE_DEVICE = 0xe,
 	SW_PEN_INSERTED = 0xf,
 };