arm64: increase MAXCPU to 1024, following amd64

As in commit 9051987e40c5 for amd64, support up to 1024 CPU cores.
arm64 hardware with more than 256 CPU cores is currently available and
will become increasingly common over FreeBSD 14's lifetime.

PR: 269572
Reviewed by: andrew
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41319