gpio: ixp4xx: Convert to hierarchical GPIOLIB_IRQCHIP

This modifies the IXP4xx driver to use the new helpers
to handle the remapping of parent to child hardware irqs
in the gpiolib core.

This pulls the majority of the code out of the driver
and use the generic code in gpiolib.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Lina Iyer <ilina@codeaurora.org>
Cc: Jon Hunter <jonathanh@nvidia.com>
Cc: Sowjanya Komatineni <skomatineni@nvidia.com>
Cc: Bitan Biswas <bbiswas@nvidia.com>
Cc: linux-tegra@vger.kernel.org
Cc: Thierry Reding <treding@nvidia.com>
Cc: Brian Masney <masneyb@onstation.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20190808123242.5359-2-linus.walleij@linaro.org