dt: Document Qualcomm APQ8084 pinctrl binding

Define a new binding for the Qualcomm TLMM (Top-Level Mode Mux) based pin
controller inside the APQ8084.

Acked-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Georgi Djakov <gdjakov@mm-sol.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>