powerpc/pseries: Add driver for PAPR SCM regions

Adds a driver that implements support for enabling and accessing PAPR
SCM regions. Unfortunately due to how the PAPR interface works we can't
use the existing of_pmem driver (yet) because:

 a) The guest is required to use the H_SCM_BIND_MEM h-call to add
    add the SCM region to it's physical address space, and
 b) There is currently no mechanism for relating a bare of_pmem region
    to the backing DIMM (or not-a-DIMM for our case).

Both of these are easily handled by rolling the functionality into a
seperate driver so here we are...

Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>