ARM: hw_breakpoint: trap undef instruction exceptions in reset_ctrl_regs

The ARM debug registers can only be accessed if the DBGSWENABLE signal
to the core is driven HIGH by the DAP. The architecture does not provide
a way to detect the value of this signal, so the best we can do is
register an undef_hook to trap debug register co-processor accesses and
then fail if the trap is taken.

Signed-off-by: Will Deacon <will.deacon@arm.com>