ARM: gic: allow GIC to support non-banked setups

The GIC support code is heavily using the fact that hardware
implementations are exposing banked registers. Unfortunately, it
looks like at least one GIC implementation (EXYNOS) offers both
the distributor and the CPU interfaces at different addresses,
depending on the CPU.

This problem is solved by allowing the distributor and CPU interface
addresses to be per-cpu variables for the platforms that require it.
The EXYNOS code is updated not to mess with the GIC internals while
handling interrupts, and struct gic_chip_data is back to being private.
The DT binding for the gic is updated to allow an optional "cpu-offset"
value, which is used to compute the various base addresses.

Finally, a new config option (GIC_NON_BANKED) is used to control this
feature, so the overhead is only present on kernels compiled with
support for EXYNOS.

Tested on Origen (EXYNOS4) and Panda (OMAP4).

Cc: Kukjin Kim <kgene.kim@samsung.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Thomas Abraham <thomas.abraham@linaro.org>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>