Add pin configuration and pinmux support for UniPhier ProXstream2
SoC.

Changes in v2:
  - sort groups and funcs alphabetically
  - add i2c pin-mux settings
  - sort members of platform_driver
  - change to tristate
  - add THIS_MODULE to pinctrl_desc
  - use module_platform_driver
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>