pinctrl: Add support for msm8917 pinctrl

This implementation is based on snapshot of the msm8917
pinctrl driver from msm-3.18 commit c12dc85c6dace
("Update target name references for MSM8917").
Also add a new document of msm8917 pinctrl bindings
according to latest pinctrl driver style.

Change-Id: Icd93372a5658f7ad480c228c44f91a00de2f000b
Signed-off-by: Swetha Chikkaboraiah <schikk@codeaurora.org>