ARM GIC-600 IP complies with ARM GICv3 architecture, but among others,
implements a power control register in the Redistributor frame. This
register must be programmed to mark the frame as powered on, before
accessing other registers in the frame. Rest of initialization sequence
remains the same.

The driver provides APIs for Redistributor power management, and
overrides those in the generic GICv3 driver. The driver data is shared
between generic GICv3 driver and that of GIC-600.

For FVP platform, the GIC-600 driver is chosen when FVP_USE_GIC_DRIVER
is set to FVP_GIC600. Also update user guide.

Change-Id: I321b2360728d69f6d4b0a747b2cfcc3fe5a20d67
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>