Add support for nRF5x series GPIOTE based PWM driver
implementation.
Provides upto 3 pins/channels using one HF timer, two PPI
channels per pin, and one GPIOTE config per pin.
Change-id: I6056b199ec2cff595ba8fea9f659a0338ed4635b
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>