[POWERPC] iSeries: Move detection of virtual tapes

Now we will only have entries in the device tree for the actual existing
devices (including their OS/400 properties).  This way viotape.c gets
all the information about the devices from the device tree.

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