platform: msm: add support for PWM mode

QPNP vibrator operates in two modes - manual and
PWM (Pulse Width Modulation). PWM is supported
through one of dtest lines connected to the
vibrator. Add support for PWM mode by parsing
channel, period and duty cycle information
from device tree.

Change-Id: I2ba5c1592dfce75d5e8dedd247d1e91c2931a832
Signed-off-by: Mohan Pallaka <mpallaka@codeaurora.org>