ppc64 iSeries: Move iSeries ppc_md functions into a machdep_calls struct

Move the iSeries machine specific calls into a machdep_calls struct like
other platforms, rather than setting members of ppc_md explicitly.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>