arm64/sve: Add documentation

This patch adds basic documentation of the user/kernel interface
provided by the for SVE.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Alex Bennée <alex.bennee@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alan Hayward <alan.hayward@arm.com>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>