[arm64] Bump MAXMEMDOM value to 8 to match amd64

On some of the server-grade ARM64 machines the number of NUMA domains is higher
than 2. When booting GENERIC kernel on such machines the SRAT parser fails
leaving the system with a single domain. To make GENERIC kernel usable on those
server, match the parameter value with the one for amd64 arch.

Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D27368
Sponsored by: Ampere Computing
Submitted by: Klara, Inc.