platform/x86: intel-vbtn: support panel front button

platform/x86: intel-vbtn: support panel front button

The Lenovo Helix 2 and Dell XPS 12 (9Q33) have an extra button on the
front showing a 'Windows' logo, both reporting event codes 0xC2/0xC3
on press/release. On the Dell, both press/release are distinct events
while on the Helix 2 both events are generated on release.

Tested on XPS 12, for info on the Helix 2 see:
https://www.spinics.net/lists/ibm-acpi-devel/msg03982.html

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>