drivers: pwm: Add driver for xmc4xxx using ccu4 module

Adds driver for pwm on xmc4xxx using Capture Compare Unit 4 (CCU4)
module. There are four CCU4 with each one having four channels
Thus it's possible to have up to 16 pwm output signals. The output of
each channel can only be connected to a specific port/pin. The possible
connection and gpio configurations are defined using pinctrl.

The CCU4 module also has a capture mode. Capture support will be added
in the future.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>