Input: add driver for SOC button array

This patch adds support for the GPIO buttons on some Intel Bay Trail
tablets originally running Windows 8. The ACPI description of these
buttons follows "Windows ACPI Design Guide for SoC Platforms".

Signed-off-by: Lejun Zhu <lejun.zhu@linux.intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>