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>

---

Changes since v2
----------------

Changes requested by Alan Hayward:

 * Added a note that the caller of PTRACE_SETREGSET will need to do a
   GETREGSET if complete certainty about the resulting VL is desired.

ABI changes:

 * Documented the changed return value value semantics for PR_SET_SET_VL
   when the PR_SVE_SET_VL_ONEXEC flag is passed.