Add single-register MMIO GPIO driver for complex cases where
only several fields in register belong to GPIO lines and each GPIO
line owns a field with different length and on/off value.

Such CREG GPIOs are used in Synopsys AXS10x and HSDK boards.
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>