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>
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>

---

Changes since v3
----------------

Changes requested by Catalin Marinas

 * Typo fix: SIG_SVE_ -> SVE_SIG_ in sigcontext macro names.