platform/x86: intel-vbtn: support SW_TABLET_MODE

Event code 0xcc is emitted by several convertibles (Dell XPS 12 9Q33 BIOS
A8, Dell XPS 13 2in1 9365, HP Spectre x360, Lenovo Thinkpad Helix) when
entering tablet mode, and 0xcd on return to laptop mode.

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