pinctrl: mdm9640: Add pinctrl driver for mdm9640

Add pinctrl driver for MDM9640 by providing the
hardware description for pins on the MDM9640
chipset. Also update the documentation with the
pinctrl compat string.

Change-Id: I415450c00979d7fe0243d019472273cfa666f330
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>