pinctrl: Device tree bindings for Qualcomm PMIC MPP block

DeviceTree binding documentation for Qualcomm SPMI PMIC MPP
pinctrl drivers.

Signed-off-by: Ivan T. Ivanov <iivanov@mm-sol.com>
Acked-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>