arm/gic-v3: Do early GICD ioremap and clean up

arm/gic-v3: Do early GICD ioremap and clean up

For ACPI based XEN boot, the GICD region needs to be accessed inside
the function gicv3_acpi_init() in later patch. There is a duplicate
panic() message, one in the DTS probe and second one in the ACPI probe
path. For these two reasons, move the code that validates the GICD base
address and does the region ioremap to a separate function. The
following patch accesses the GICD region inside gicv3_acpi_init() for
finding per CPU Redistributor size.

Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>