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

[ Upstream commit 8d9e29972836b75eb74f533594999500a4c7cc19 ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>