arc: hsdk: add pinmux driver support and doc enhancement

* add pinmux driver. hsdk board has arduino, mikrobus and
  pmod interfaces, which can be confiured for different function,
  such as: gpio, spi, uart, iic.
* add introduction for arduino, mikrobus and pmod interfaces.

Signed-off-by: Watson Zeng <zhiwei@synopsys.com>