[POWERPC] Merge iSeries i/o operations with the rest

This patch changes the io operations so that they are out of line if
CONFIG_PPC_ISERIES is set and includes a firmware feature check in
that case.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>