Add support for the GPIO controller of the sl28 board management
controller. This driver is part of a multi-function device.

A controller has 8 lines. There are three different flavors:
full-featured GPIO with interrupt support, input-only and output-only.
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>