leds: leds-qpnp: PWM period granularity auto selection for leds

Some leds use PWM wave to control the brightness. The original
period granularity is using microsecond by default, it's a very
large granularity and will cause some user experience issues on
leds' brightness.

Auto select the PWM period granularity between microsecond and
nanosecond according to the range that hardware can support.

CRs-Fixed: 622683
Change-Id: I0669a0c88ca2ba491f0a53be6070476d61374d72
Signed-off-by: Xu Kai <kaixu@codeaurora.org>