KVM: arm64: Add reg_feat_map_desc to describe full register dependency

KVM: arm64: Add reg_feat_map_desc to describe full register dependency

struct reg_bits_to_feat_map is great to describe bit-to-feature
dependency, but not so much to describe register-to-feature
dependency. Yet both need to exist.

Add a new reg_feat_map_desc structure to describe this.

Extra complexity is added by the need to source the RES0 bits from
the runtime-computed FGT masks, for which we need an extra flag
and extra complexity. Oh well.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>