powerpc: Add missing DABR flags

The powerpc 64 bit architecture defines three flags for the
DABR (Data Address Breakpoint Register).  Add definitions
for the currently missing DABR_DATA_WRITE and DABR_DATA_READ
flags to the powerpc reg.h file.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>