powerpc: Update Integrated Flash controller device tree bindings

Freescale's Integrated Flash controller (IFC) may have one or two
interrupts. In case of single interrupt line, it will cover all IFC
interrupts.

Update this information in IFC device tree bindings

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>