ARM: hw_breakpoint: don't try to clear v6 debug registers during boot

v6 cores do not provide a way to clear the debug registers without first
enabling monitor mode, meaning that we could take spurious debug
exceptions. Instead, rely on the registers being in a sane state when we
boot as they are defined to be disabled out of reset anyway.

Tested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>