pinctrl: driver for Conexant Digicolor CX92755 pin mapping

pinctrl: driver for Conexant Digicolor CX92755 pin mapping

This adds pinctrl and gpio driver to the CX92755 SoC "General
Purpose Pin Mapping" hardware block. The CX92755 is one SoC
from the Conexant Digicolor series. Pin mapping hardware supports
configuring pins as either GPIO, or up to 3 other "client select"
functions. This driver adds support for pin muxing using the
generic device tree binding, and a basic gpiolib driver for
the GPIO functionality.

This driver does not currently support GPIO interrupts, and
pad configuration.

v2:
* Address review comments for Linus Walleij:
  - Add a pointer to pinctrl_desc in struct dc_pinmap
  - Drop the now redundant pinctrl_pin_desc field
  - Adapt dc_get_group_{name,pins} to these changes, and
    add a comment explaining the 1-to-1 pin-groups relation
* Staticise dc_pmxops
* Protect the GP_CLIENTSEL clct parameter with parenthesis

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>