powerpc: Make coff boot wrapper load the kernel at 8M

powerpc: Make coff boot wrapper load the kernel at 8M

Previously it claimed 7MB starting at the 9M point and loaded the
kernel there.  That meant that prom_init put the flattened device
tree above 16M.  On the 601 that caused the early device tree scan
to fail, since only 16MB are mapped with BATs on the 601.  Moving
this down to 8MB allows prom_init to put the flattened device tree
between 15M and 16M, so it works on the 601.

Signed-off-by: Paul Mackerras <paulus@samba.org>