dt-bindings: gpio: brcm: Add bindings for xgs-iproc

This GPIO controller is present on a number of Broadcom switch ASICs
with integrated SoCs. It is similar to the nsp-gpio and iproc-gpio
blocks but different enough to require a separate driver.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191028200555.27524-1-chris.packham@alliedtelesis.co.nz
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>