Add the documentation for the Microsemi Ocelot pinmuxing and gpio
controller.

Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>