8042 keyboard: power button processing

power button is usually dropped because it's not
in the keyboard matrix range.  Adding in condition
to forward it like other keys.

BUG=chrome-os-partner:61275
BRANCH=None
TEST=reboot and make sure power button selection
     in detachable menus is processed on reef.

Change-Id: I516a0043bd7730789728d5c5498d0a0f30a2acac
Signed-off-by: Shelley Chen <shchen@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/428199
Reviewed-by: Randall Spangler <rspangler@chromium.org>

payloads/libpayload/drivers/keyboard.c

@@ -32,6 +32,7 @@
 #include <libpayload.h>
 
 #define I8042_CMD_DIS_KB     0xad
+#define POWER_BUTTON         0x90
 
 struct layout_maps {
 	const char *country;
@@ -255,6 +256,9 @@ int keyboard_getchar(void)
 		}
 	}
 
+	if (ch == 0x5e)
+		ret = POWER_BUTTON;
+
 	return ret;
 }