[POWERPC] iSeries: We need vio_enable_interrupts

Commit 3d0e91f7 introduced a requirement
for vio_enable_interrupts which iSeires has never needed.  So create a
dummy one.

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