arch: Add uReset helper to UPCState

arch: Add uReset helper to UPCState

This allows to reset without advancing the pc

https://gem5.atlassian.net/browse/GEM5-587

Change-Id: Ied566f4cd5efed5eb500447d3f14388482435475
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30315
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Alexandru Duțu <alexandru.dutu@amd.com>