sh: SH-DMAC compile fixes

This patch does the following:
- remove the make_ipr_irq stuff from dma-sh.c and replace it
  with a simple channel<->irq mapping table.
- add DMTEx_IRQ constants for sh4 cpus
- fix sh7751 DMAE irq number

The SH7780 uses the same IRQs for DMA as other SH4 types, so
I put the constants on top of the dma.h file.

Other CPU types need to #define their own DMTEx_IRQ contants
in their appropriate header.

Signed-off-by: Manuel Lauss <mano@roarinelk.homelinux.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>