pinctrl: Add support for msm8937 pinctrl

This implementation is based on snapshot of the msm8937
pinctrl driver from msm-3.18 commit e23f09a2bb9cf
("Update target name references for MSM8937").
Also add a new document of msm8937 pinctrl bindings
according to latest pinctrl driver style.

Change-Id: I99b4c61a849b3933cb3a18bf6550135ee6285bd3
Signed-off-by: Jingbiao Lu <jingbiao@codeaurora.org>