gpio: of: Add support for multiple GPIOs in a single GPIO hog

When listing multiple GPIOs in the "gpios" property of a GPIO hog, only
the first GPIO is affected.  The user is left clueless about the
dysfunctioning of the other GPIOs specified.

Fix this by adding and documenting support for specifying multiple
GPIOs in a single GPIO hog.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>