ARM: mach-shmobile: Use evt2irq() for sh7372 DMAC

Use evt2irq() for DMAC resources on sh7372.

This makes the code easier to read and also
allows adjusting the INTC base offset without
breaking platform data.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>