xen/arm: gic: acpi: Guard helpers to build the MADT with CONFIG_ACPI

xen/arm: gic: acpi: Guard helpers to build the MADT with CONFIG_ACPI

gic_make_hwdom_madt() and gic_get_hwdom_madt_size() are ACPI specific.

While they build fine today, this will change in a follow-up patch.
Rather than trying to fix the build on ACPI, it is best to avoid
compiling the helpers and the associated callbacks when CONFIG_ACPI=n.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>