drivers: pwm: Add driver for xmc4xxx using ccu8 module

Adds driver for pwm on xmc4xxx using Capture Compare Unit 8 (CCU8)
module. There are two CCU8 nodes with each one having four slices.
Each slice has two output channels.

Unlike CCU4, this module can generate complementary high-side/low-side
signals for each output channel. A variable dead time can be added
during the off to on transitions to make sure that the
high-side/low-side signals are not on at the same time.

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