arm64: Always set the debug control and value regs

When listing watchpoints we read the raw registers. To ensure we print
an accurate list always set the watchpoint and breakpoint registers.

Sponsored by: Arm Ltd

Reviewed by: jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D44351

(cherry picked from commit 2e2c983d5234cb5457548fc75608d93b2309b254)