platform/x86: Add support for Basin Cove power button

This provides a new input driver for supporting the power button on
Basin Cove PMIC, found on Intel Merrifield-based devices.

The driver follows the design used in intel_chtdc_ti_pwrbtn.c module.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>