drivers: pwm: always use nanoseconds for set

In order to be consistent with what is possible in Devicetree, always
take a period in nanoseconds. Other scales or units may be specified by
using, e.g., the PWM_MSEC() macros (all of them converting down to
nanoseconds). This change then deletes the "_nsec" and "_usec" versions
of the pwm_set call.

Note that this change limits the period to UINT32_MAX nanoseconds,
~4.3s. PWM is, in generali, used with periods below the second so it
should not be a problem.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>