powerpc: Fix PPC_PTRACE_SETHWDEBUG on PPC_BOOK3S

Properly set the DABR_TRANSLATION/DABR_DATA_READ/DABR_DATA_READ bits in
the dabr when setting the debug register via PPC_PTRACE_SETHWDEBUG.  Also
don't reject trigger type of PPC_BREAKPOINT_TRIGGER_READ.

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>