leds: leds-qpnp: provide pwm period when leds are set to LPG mode in DT

In the current design, if MPP-controlled leds or RGB leds are configured
to LPG mode in the device tree, the PWM period value is not provided.
This will result in "pwm_cfg->pwm_period_us" being assigned to zero
since no PWM period value will be read from device tree. As a result,
qpnp_pwm_init() returns error, which in turn results in failure of
qpnp_mpp_init() or qpnp_rgb_init() and finally resulting in failure of
qpnp_leds_probe().

So "qcom,pwm-us" property should be provided in DT for the above case and
the property parsing needs to be implemented in the leds-qpnp driver.

Change-Id: Ic32833ea4e685939148e38cdba078391936f8efb
CRs-Fixed: 667825
Signed-off-by: Abinaya P <abinayap@codeaurora.org>