pwm: add period write replace hook

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>
pwm: move internal context structure to internal header

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>
Galileo Gen2: add replace hook for pwm period

* Warning, doing a period call on the gen2 will change it for all pwm
* channels and temporarily disable them whilst doing it.

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>