x86/platform/intel-mid: Enable GPIO keys on Merrifield

x86/platform/intel-mid: Enable GPIO keys on Merrifield

The Merrifield firmware provides 3 descriptions of buttons connected to GPIO.
Append them to the list of supported GPIO keys.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170105161717.115261-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>