powerpc: Fix interrupt-tree parsing

The interrupt-tree parsing code wasn't offsetting interrupt numbers
by 16 on 32-bit platforms with an i8259 interrupt controller, and
it was confused about the encoding of interrupt sense and level
(which is different for i8259 and openpic interrupt controllers,
just to make things interesting).

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