gpio: langwell: add Intel Merrifield support

This patch implements a better way to handle multiple SoC's and adds
Intel Merrifield support to gpio-langwell.

It was based on previous work from Ning Li <ning.li@intel.com>

Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Fei Yang <fei.yang@intel.com>
Signed-off-by: Ning Li <ning.li@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>