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This patch moves the private GIC common accessors from `gic_common.h` to
a new private header file `gic_common_private.h`. This patch also adds
additional comments to GIC register accessors to highlight the fact
that some of them access register values that correspond to multiple
interrupt IDs. The convention used is that the `set`, `get` and `clr`
accessors access and modify the values corresponding to a single interrupt
ID whereas the `read` and `write` GIC register accessors access the raw
GIC registers and it could correspond to multiple interrupt IDs depending
on the register accessed.

Change-Id: I2643ecb2533f01e3d3219fcedfb5f80c120622f9

This patch adds a driver for ARM GICv2 systems, example GIC-400. Unlike
the existing GIC driver in `include/drivers/arm/arm_gic.h`, this driver
is optimised for GICv2 and does not support GICv3 systems in GICv2
compatibility mode. The driver interface has been implemented in
`drivers/arm/gic/v2/gicv2_main.c`. The corresponding header is in
`include/drivers/arm/gicv2.h`. Helper functions are implemented in
`drivers/arm/gic/v2/gicv2_helpers.c` and are accessible through the
`drivers/arm/gic/v2/gicv2_private.h` header.

Change-Id: I09fffa4e621fb99ba3c01204839894816cd89a2a