powerpc/irq: Add mechanism to force a replay of interrupts

Calling this function with interrupts soft-disabled will cause
a replay of the external interrupt vector when they are re-enabled.

This will be used by the OPAL XICS backend (and latter by the native
XIVE code) to handle EOI signaling that there are more interrupts to
fetch from the hardware since the hardware won't issue another HW
interrupt in that case.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>