cpu: Add hash functionality for RegId class

Having a hash function defined within the header will allow all
classes using RegId to use the class as a Key in a STL
unordered_map.

Change-Id: I32fd302a087c74e844dcbfce93fef9d0ed98d6bf
Signed-off-by: Bradley Wang <radwang@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/11870
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>