ssbi-gpio did not have any irqchip support so consumers of this in
device tree would need to call gpio[d]_to_irq() in order to get the
proper IRQ on the underlying PMIC. IRQ chips in device tree should
be usable from the start without the consumer having to make an
additional call to get the proper IRQ on the parent. This patch adds
hierarchical IRQ chip support to the ssbi-gpio code to correct this
issue.

The constant PM8XXX_GPIO_PHYSICAL_OFFSET is introduced to replace the
hardcoded '1' that previously existed in two places in this driver to
improve code readability.

This change was tested on an APQ8060 DragonBoard.
Signed-off-by: Brian Masney <masneyb@onstation.org>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>