arch-arm: Fix usage of RegId constructor for VecElem

arch-arm: Fix usage of RegId constructor for VecElem

RegId() constructor requires three arguments in case of a VecElem
register: the class, the vector index and the element index inside the
vector, otherwise it panics.

Change-Id: Ic842df4dcddaffa83b211aa6e1dd1953cafa4951
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15615
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>