powerpc/xive: add the HW IRQ number under xive_irq_data

It will be required later by the H_INT_ESB hcall.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>