gic_acpi: Limit the number of CPUs to GIC_MAXCPU

madt_table_data contains an array of pointers for each CPU and was
allocated on the stack. If MAXCPU is raised to a sufficiently large
value this can overflow the kernel stack. Cap the stack growth by
using GIC_MAXCPU instead as for other parts of the gicv1/v2 driver in
commit a0e20c0ded1a.

Suggested by: andrew
Reviewed by: andrew, emaste
Obtained from: CheriBSD
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D41800