pwm: qti-lpg: Add config_extend() to set longer PWM period

Add config_extend() hook function to set PWM period/duty parameters
with u64 data type which can support longer PWM period/duty length.

Change-Id: I39e81463104bc2361731167ba98a28470804f192
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>