powerpc: Fix interrupt values for DMA2 in MPC8610 HPCD device tree

For Freescale 8xxx devices that use an MPIC, the interrupt numbers in
the device tree must be 16 greater than the values documented in the
reference manual.  In these chips, the MPIC is wired to use the first
16 numbers for external interrupts, but the documentation numbers
internal interrupts from 0.

In the MPC8610 HPCD device tree, the interrupt properties for the DMA
channels for DMA2 were not the adjusted values.  This fixes that.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>