pinctrl: Add Pistachio SoC pin control driver

Add a driver for the pin controller present on the IMG Pistachio SoC.
This driver provides pinmux and pinconfig operations as well as GPIO
and IRQ chips for the GPIO banks.

Changes from v4:
 - Switched to using gpiochip_add_pin_range().
 - Fixed up Kconfig entry.
Changes from v3:
 - Addressed review comments from Ezequiel.
Changes from v2:
 - Removed module stuff which would be compiled out.
Changes from v1:
 - Addressed review comments from Linus.
 - Changed compatible string to "img,pistachio-system-pinctrl".
 - Look for GPIO sub-nodes by name.
 - A couple of bug fixes.

Signed-off-by: Damien Horsley <Damien.Horsley@imgtec.com>
Signed-off-by: Govindraj Raja <govindraj.raja@imgtec.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Signed-off-by: Kevin Cernekee <cernekee@chromium.org>
Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: James Hartley <james.hartley@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>