powerpc: Add helper functions set the DAWR and CIABR using set_mode

These are just wrappers around the new set_mode HCALL.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>