ioctl definitions and related datatypes that allow userland control of pwm
hardware via the pwmc device. The new name and location better reflects its
assocation with a single device driver.
The pwm subsystem consist of API for PWM controllers, pwmbus to register them
and a pwm(8) utility to talk to them from userland.
Reviewed by: oshgobo (capsicum), bcr (manpage), 0mp (manpage)
Differential Revision: https://reviews.freebsd.org/D17938
