dt: pwm: add support for specifying frequency in Hz and kHz

Add a pair of dt macros for specifying the pwm frequency in hertz or
kilohertz: PWM_HZ and PWM_KHZ. This is then converted in period
nanoseconds so it works as expected with the other definitions.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>