dt-bindings: pwm: Add DT bindings for QTI PWM LPG driver

QTI PWM LPG driver supports configuring PWM/LPG channels on QTI
PMICs based on the client's request. Add snapshot of device tree
bindings for it.

This snapshot is taken as of msm-4.14
commit 9bb584ae ("msm/sde/rotator: Add rev checks for sdmmagpie").

Change-Id: I4640065c17cba28a64bcdca4646cbe2471e986cc
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>