irqchip/gic-v3: Gather all ACPI specific data in a single structure

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

To make clear those variables are ACPI specific, gather all of them in a
single structure.

Furthermore, even if some of the variables are not marked with
__initdata, they are all only used during the initialization. Therefore,
the new variable, which hold the structure, can be marked with
__initdata.

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