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GICD_IPRIORITYR and GICD_ITARGETSR specifically support byte addressing
so that individual interrupt priorities can be atomically updated by
issuing a single byte write. The previous implementation of
gicd_set_ipriority() and gicd_set_itargetsr() used 32-bit register
accesses, modifying values for 4 interrupts at a time, using a
read-modify-write approach. This potentially may cause concurrent changes
by other CPUs to the adjacent interrupts to be corrupted. This patch fixes
the issue by modifying these accessors to use byte addressing.

Fixes ARM-software/tf-issues#343

Change-Id: Iec28b5f5074045b00dfb8d5f5339b685f9425915

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