i2c: add nexell driver

i2c: add nexell driver

Changes in relation to FriendlyARM's U-Boot nanopi2-v2016.01:
- i2c/nx_i2c.c: Some adaptions mainly because of changes in
  "struct udevice".
- several Bugfixes in nx_i2c.c.
- the driver has been for s5p6818 only. Code extended appropriately
  in order s5p4418 is also working.
- "probe_chip" added.
- pinctrl-driver/dt is used instead of configuring the i2c I/O-pins
  in the i2c-driver.
- '#ifdef CONFIG...' changed to 'if (IS_ENABLED(CONFIG...))' where
  possible (and similar).
- livetree API (dev_read_...) is used instead of fdt one (fdt...).

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