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: Alan Hayward <alan.hayward@arm.com>
Cc: Alex Bennée <alex.bennee@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: linux-api@vger.kernel.org
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/uapi/asm/sigcontext.h

@@ -133,6 +133,9 @@ struct sve_context {
  * The SVE architecture leaves space for future expansion of the
  * vector length beyond its initial architectural limit of 2048 bits
  * (16 quadwords).
+ *
+ * See linux/Documentation/arm64/sve.txt for a description of the VL/VQ
+ * terminology.
  */
 #define SVE_VQ_BYTES		16	/* number of bytes per quadword */