powerpc/opal: Add PHB to cxl mode call

This adds the OPAL call to change a PHB into cxl mode.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>