drivers: pwm: remove _pin from API calls

In a first place, the PWM API operates on "channels", not "pins". While
the API calls could have been changed by _channel, this patch takes the
approach of just dropping _pin. The main reason is that all API calls
operate by definition on a channel basis, so it is a bit redundant to
make this part of the name. Because the `_dt` variants of the calls are
going to be introduced soon, the change to `_channels` + `_dt` would
make API function names quite long.

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