Adding a pwm command

[Marc Reilly]

Hi all, 

I've been playing around with adding a new board to barebox, and its
been a while so I didn't realise that properties could be set on a
driver instance directly (eg 'pwm0.enable=1' or 'pwm0.period_ns=10000')

I started this pwm_set command, if it's useful (ability to set frequency
is helpful) I can submit it properly. It looks like most other commands
have doc duplicated in the Kconfig, any other requests?

Cheers
Marc