irqchip/gic-v2: Gather ACPI specific data in a single structure

The ACPI code requires to use global variables in order to collect
information from the tables.

For now, a single global variable is used, but more will be added in a
subsequent patch. To make clear they are ACPI specific, gather all the
information in a single structure.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Christofer Dall <christoffer.dall@linaro.org>
Acked-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>