arm64/sve: In-kernel vector length availability query interface

KVM will need to interrogate the set of SVE vector lengths
available on the system.

This patch exposes the relevant bits to the kernel, along with a
sve_vq_available() helper to check whether a particular vector
length is supported.

vq_to_bit() and bit_to_vq() are not intended for use outside these
functions, so they are given a __ prefix to warn people not to use
them unless they really know what they are doing.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>