irqchip/exiu: Preparatory refactor for ACPI support

In preparation of adding support for EXIU controller devices described
via ACPI, split the DT init function in a DT specific and a generic part,
where the latter will be reused for ACPI support later.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>