arm64/sve: Clarify vq map semantics

Currently the meanings of sve_vq_map and the ancillary helpers
__bit_to_vq() and __vq_to_bit() are not clearly explained.

This patch makes the explanatory comment clearer, and removes the
duplicate comment from fpsimd.h.

The WARN_ON() currently present in __bit_to_vq() confuses the
intended use of this helper.  Since these are low-level helpers not
intended for general-purpose use anyway, it is better not to make
guesses about how these functions will be used: rather, this patch
removes the WARN_ON() and relies on callers to use the helpers
sensibly.

Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>