[PATCH] powerpc: cleanup of iSeries flat device tree

Consolidate the vio device node creation.  Make some parameters const.
Make a few more things __initdata. Get the device_type strings out of
the device tree blob.

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