mfd: Add support for Merrifield Basin Cove PMIC

Add an MFD driver for Intel Merrifield Basin Cove PMIC.

Firmware on the platforms which are using Basin Cove PMIC is "smarter"
than on the rest supported by vanilla kernel. It handles first level
of interrupt itself, while others do it on OS level.

The driver is done in the same way as the rest of Intel PMIC MFD drivers
in the kernel to support the initial design. The design allows to use
one driver among few PMICs without knowing implementation details of
the each hardware version or generation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>