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>