pwm: add driver for nexell

Changes in relation to FriendlyARM's U-Boot nanopi2-v2016.01:
- Since drivers/pwm/pwm-nexell.c is an adapted version of
  s5p-common/pwm.c an appropriately changed version of s5p-common/pwm.c
  is used instead. Therefore arch/arm/mach-s5pc1xx/include/mach/pwm.h
  copied to arch/arm/mach-nexell/include/mach and s5p-common/Makefile
  changed appropriately.
- '#ifdef CONFIG...' changed to 'if (IS_ENABLED(CONFIG...))' where
  possible (and similar).

Signed-off-by: Stefan Bosch <stefan_b@posteo.net>