gpio: bcm281xx: Add GPIO driver

Add the GPIO driver for the Broadcom bcm281xx family of mobile SoCs.
These GPIO controllers may contain up to 8 banks where each bank
includes 32 pins that can be driven high or low and act as an edge
sensitive interrupt.

Signed-off-by: Markus Mayer <markus.mayer@linaro.org>
Reviewed-by: Christian Daudt <csd@broadcom.com>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Matt Porter <matt.porter@linaro.org>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
[Added depends on OF_GPIO]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>