arm64: default NR_CPUS to 256

There are shipping arm64 platforms with 256 hardware threads. So that we
can make use of these with defconfig, bump the arm64 default NR_CPUS to
256.

At the same time, drop a redundant comment. We only have one default for
NR_CPUS, so there's nothing to sort.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>