drivers: pwm: pwm_mcux_sctimer: support reconfiguring period

Add support for reconfiguring the period of an active PWM channel with
the SCTimer driver. Due to the design of the SCTimer IP, the period can
only be reconfigured when one channel is in use. Otherwise, only the
duty cycle of each channel can be updated.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>