arm64: docs: describe ELF hwcaps

We don't document our ELF hwcaps, leaving developers to interpret them
according to hearsay, guesswork, or (in exceptional cases) inspection of
the current kernel code.

This is less than optimal, and it would be far better if we had some
definitive description of each of the ELF hwcaps that developers could
refer to.

This patch adds a document describing the (native) arm64 ELF hwcaps.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Martin <Dave.Martin@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
[ Updated new hwcap entries in the document ]
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>