pwm: sti: Reorganise register names in preparation for new functionality

Exciting functionality is on the way to this device.  But
before we can add it, we need to do some basic housekeeping
so the additions can be added cleanly.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>