intel-vbtn: new driver for Intel Virtual Button

This driver supports power button event in Intel Virtual Button currently.
New Dell XPS 13 requires this driver for the power button.

This driver is copied/modified from intel-hid.c
Most credit goes to the author of intel-hid.c,
Alex Hung <alex.hung@canonical.com>

Signed-off-by: AceLan Kao <acelan.kao@canonical.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>