arm: add mach-nexell (all files except header files)

arm: add mach-nexell (all files except header files)

Changes in relation to FriendlyARM's U-Boot nanopi2-v2016.01:
- SPL not supported yet --> no spl-directory in arch/arm/mach-nexell.
  Appropriate line in Makefile removed.
- clock.c: 'section(".data")' added to declaration of clk_periphs[] and
  core_hz.
- Kconfig: Changes to have a structure like in mach-bcm283x/Kconfig,
  e.g. "config ..." entries moved from other Kconfig.
- timer.c: 'section(".data")' added to declaration of timestamp and
  lastdec.
- arch/arm/mach-nexell/serial.c removed because this is for the UARTs
  of the S5P6818 SoC which is not supported yet. S5P4418 UARTs are
  different, here the (existing) PL011-code is used.
- '#ifdef CONFIG...' changed to 'if (IS_ENABLED(CONFIG...))' where
  possible (and similar).

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