[arm64] Bump MAXMEMDOM value to 8 to match amd64

[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.