[POWERPC] Fix new interrupt code (MPIC detection)

As the code comment already says, the Maple device-tree is incorrect here;
make the Linux code detect the correct thing, too.

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