arm64: make flatmem depend on !NUMA

Building without NUMA but with FLATMEM results in a link error
because mem_map[] is not available:

aarch64-linux-ld -EB -maarch64elfb --no-undefined -X -pie -shared -Bsymbolic --no-apply-dynamic-relocs --build-id -o .tmp_vmlinux1 -T ./arch/arm64/kernel/vmlinux.lds --whole-archive built-in.a --no-whole-archive --start-group arch/arm64/lib/lib.a lib/lib.a --end-group
init/do_mounts.o: In function `mount_block_root':
do_mounts.c:(.init.text+0x1e8): undefined reference to `mem_map'
arch/arm64/kernel/vdso.o: In function `vdso_init':
vdso.c:(.init.text+0xb4): undefined reference to `mem_map'

This uses the same trick as the other architectures, making flatmem
depend on !NUMA to avoid the broken configuration.

Fixes: e7d4bac4 ("arm64: add ARM64-specific support for flatmem")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>