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A GICv3 interrupt controller will be instantiated for a certain number
of cores. This will result in the respective number of GICR frames. The
last frame will have the "Last" bit set in its GICR_TYPER register.

For platforms with a topology unknown at build time (the Arm FPGAs, for
instance), we need to learn the number of used cores at runtime, to size
the GICR region in the devicetree accordingly.

Add a generic function that iterates over all GICR frames until it
encounters one with the "Last" bit set. It returns the number of cores
the GICv3 has been configured for.

Change-Id: I79f033c50dfc1c275aba7122725868811abcc4f8
Signed-off-by: Andre Przywara <andre.przywara@arm.com>