powerpc: Add wrappers to enable/disable relocation on exceptions

These wrappers hide the parameters that have to be passed to H_SET_MODE
to enable/disable relocation on during exceptions.

As noted in the comments, since these have partition wide scope, they
may take some time to complete and must be periodically retried until
H_SUCCESS is returned.

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>