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>

---

Changes since v1
----------------

Requested by Alex Bennée:

* Add cross-reference from sigcontext.h to the description of VL/VQ.

Other:

* [ABI fix] Bail out with -EIO if attempting to set the SVE regs for an
unsupported VL, rather than misparsing the regset data.

* Document detection of SVE via CPUID MRS.

* Remove comment about coredump notes being padded for NT_ARM_SVE.
This is no longer the case with variable-size regsets supported
in the core code.

* Fix a missing bullet char