Restructure the pwm device hirearchy and interfaces.

The pwm and pwmbus interfaces were nearly identical, this merges them into a
single pwmbus interface. The pwmbus driver now implements the pwmbus
interface by simply passing all calls through to its parent (the hardware
driver). The channel_count method moves from pwm to pwmbus, and the
get_bus method is deleted (just no longer needed).

The net effect is that the interface for doing pwm stuff is now the same
regardless of whether you're a child of pwmbus, or some random driver
elsewhere in the hierarchy that is bypassing the pwmbus layer and is talking
directly to the hardware driver via cross-hierarchy connections established
using fdt data.

The pwmc driver is now a child of pwmbus, instead of being its sibling
(that's why the get_bus method is no longer needed; pwmc now gets the
device_t of the bus using device_get_parent()).