pwm: qpnp: provide both nsec/usec-supported pwm config() functions

Kernel 3.10 introduces pwm framework, and the standard pwm_config()
API uses nanosecond-level parameters, so we should implement it.

But Qualcomm platform should support 384 seconds pwm-period at most,
the above standard API couldn't satisfy the requirement of the bigger
period, so we also provide pwm_config_us() this such platform related
API.

CRs-Fixed: 587715
Change-Id: Ia3ea9904141f69c1f40a8619ba8d9fe891e3460a
Signed-off-by: Xu Kai <kaixu@codeaurora.org>