pwm: pwm-qpnp: add missing documentation file

Device tree binding documentation file for pwm-qpnp driver was
missed in snapshot commit f59a2a72 ("leds: qpnp: Add support
for QPNP drivers") during the kernel upgrade. Bring it in from
msm-3.10 to msm-3.18 as that is the latest.

Change-Id: I36a3a8246fba147204d750495cf9d64fa4209842
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>