arm64/sme: Document SME 2 and SME 2.1 ABI

arm64/sme: Document SME 2 and SME 2.1 ABI

As well as a number of simple features which only add new instructions and
require corresponding hwcaps SME2 introduces a new register ZT0 for which
we must define ABI. Fortunately this is a fixed size 512 bits and therefore
much more straightforward than the base SME state, the only wrinkle is that
it is only accessible when ZA is accessible.

While there is only a single register the architecture is written with a
view to exensibility, including a number in the name, so follow this in the
ABI.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-sme2-v4-4-f2fa0aef982f@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>