powerpc/watchpoint/xmon: Don't allow breakpoint overwriting

Xmon allows overwriting breakpoints because it's supported by only
one DAWR. But with multiple DAWRs, overwriting becomes ambiguous
or unnecessary complicated. So let's not allow it.

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Michael Neuling <mikey@neuling.org>
Link: https://lore.kernel.org/r/20200514111741.97993-16-ravi.bangoria@linux.ibm.com