pinctrl: Add Pistachio SoC pin control binding document

Add a device-tree binding document for the pin controller present
on the IMG Pistachio SoC.

Signed-off-by: Damien Horsley <Damien.Horsley@imgtec.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>