arm64: Reserve HWCAP2_MTE as (1 << 18)

While MTE is not supported in the upstream kernel yet, add a comment
that HWCAP2_MTE as (1 << 18) is reserved. Glibc makes use of it for the
resolving (ifunc) of the MTE-safe string routines.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>