powerpc/ipic: unmask all interrupt sources

in case the interrupt controller was used in an earlier life then it is
possible it is that some of its sources were used and are still unmask.
If the (unmasked) device is active and is creating interrupts (or one
interrupts was pending since the interrupts were disabled) then the boot
process "ends" very soon. Once external interrupts are enabled, we land in
-> do_IRQ
  -> call ppc_md.get_irq()
     -> ipic_read() gets the source number
     -> irq_linear_revmap(source)
        -> revmap[source] == NO_IRQ
           -> irq_find_mapping(source) returns NO_IRQ because no source
              is registered
  -> source is NO_IRQ, ppc_spurious_interrupts gets incremented, no
     further action.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>