powerpc/ipic: Support edge on IRQ0

powerpc/ipic: Support edge on IRQ0

External IRQ0 (index 48) has the same capabilities as the other IRQ1-7
and is handled by the same register IPIC_SEPNR. When this register is
not specified for "ack" in "ipic_info", you cannot configure this IRQ
as IRQ_TYPE_EDGE_FALLING. This oversight was probably due to the
non-contiguous hwirq numbering of IRQ0 in the IPIC.

Signed-off-by: Jurgen Schindele <schindele@nentec.de>
[scottwood: Cleaned up commit message and posted as a proper patch]
Signed-off-by: Scott Wood <oss@buserror.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>