arm64/hwcap: Document allocation of upper bits of AT_HWCAP

The top two bits of AT_HWCAP are reserved for use by glibc and the rest of
the top 32 bits are being kept unallocated for potential use by glibc.
Document this in the header.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Link: https://lore.kernel.org/r/20220707103632.12745-2-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>