Follow changes in the pwmc(4) driver in relation to device filenames.

Follow changes in the pwmc(4) driver in relation to device filenames.

The driver now names its cdev nodes pwmcX.Y where X is unit number and
Y is the channel within that unit.  Change the default device name from
pwmc0 to pwmc0.0.  The driver now puts cdev files and label aliases in
the /dev/pwm directory, so allow the user to provide unqualified names
with -f and automatically prepend the /dev/pwm part for them.

Update the examples in the manpage to show the new device name format
and location within /dev/pwm.