platform/x86: intel-vbtn: add volume up and down

Tested on HP Elite X2 1012 G1.
Matches event report of Lenovo Helix 2
(https://www.spinics.net/lists/ibm-acpi-devel/msg03982.html).
Signed-off-by: Maarten Maathuis <madman2003@shikahr.net>
[andy: fixed indentation of comments and massaged title of the change]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/platform/x86/intel-vbtn.c

@@ -37,6 +37,10 @@ static const struct acpi_device_id intel_vbtn_ids[] = {
 static const struct key_entry intel_vbtn_keymap[] = {
 	{ KE_IGNORE, 0xC0, { KEY_POWER } },	/* power key press */
 	{ KE_KEY, 0xC1, { KEY_POWER } },	/* power key release */
+	{ KE_KEY, 0xC4, { KEY_VOLUMEUP } },		/* volume-up key press */
+	{ KE_IGNORE, 0xC5, { KEY_VOLUMEUP } },		/* volume-up key release */
+	{ KE_KEY, 0xC6, { KEY_VOLUMEDOWN } },		/* volume-down key press */
+	{ KE_IGNORE, 0xC7, { KEY_VOLUMEDOWN } },	/* volume-down key release */
 	{ KE_END },
 };