[PATCH] ppc: Fix boot with yaboot with ARCH=ppc

The merge of machine types broke boot with yaboot & ARCH=ppc due to the
old code still retreiving the old-syle machine type passed in by yaboot.
This patch fixes it by translating those old numbers. Since that whole
mecanism is deprecated, this is a temporary fix until ARCH=ppc uses the
new prom_init that the merged architecture now uses for both ppc32 and
ppc64 (after 2.6.15)

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>