powerpc: Use the XDABR hcall

We never use the XDABR hcall since we check for DABR hcall first.
XDABR syscall is better since it allows us to also set the DABRX.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>