gpio: Add xgs-iproc driver

This driver supports the Chip Common A GPIO controller present on a
number of Broadcom switch ASICs with integrated SoCs. The controller is
similar to the pinctrl-nsp-gpio and pinctrl-iproc-gpio blocks but
different enough that a separate driver is required.

This has been ported from Broadcom's XLDK 5.0.3 retaining only the CCA
support (pinctrl-iproc-gpio covers CCB).

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Link: https://lore.kernel.org/r/20191024202703.8017-3-chris.packham@alliedtelesis.co.nz
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>