[PATCH] powerpc: Fix machine detection in prom_init.c

In e8222502 the detection of machine types
in prom_init broke for some machines. We should be checking /device_type
instead of /model. This should make Power3 and Power4 boot again. Haven't
been able to test this.  We also need to relocate before comparing.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>