drivers: pwm_qmsi: Do not use CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC

On some SoCs the frequency of the system clock is obtained at run time
as the exact configuration of the hardware is not known at compile time.
On such platforms using CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC define
directly introduces timing errors.

This commit replaces CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC by the call
to inline function sys_clock_hw_cycles_per_sec() which always returns
correct frequency of the system clock.

Signed-off-by: Piotr Zięcik <piotr.ziecik@nordicsemi.no>