pwm: qti-lpg: Read tick duration via device tree

Tick duration defines the tick duration (in microseconds) for the clock
that PPG uses which is used to program the ramp step duration for the
pattern. If unspecified, the default value is 7800 us.

Change-Id: I7cd96d74a39becffbf9a008610c0092ad2462446
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>