sh: multiple vectors per irq - base

Instead of keeping the single vector -> single linux irq mapping
we extend the intc code to support merging of vectors to a single
linux irq. This helps processors such as sh7750, sh7780 and sh7785
which have more vectors than masking ability. With this patch in
place we can modify the intc tables to use one irq per maskable
irq source. Please note the following:

 - If multiple vectors share the same enum then only the
   first vector will be available as a linux irq.

 - Drivers may need to be rewritten to get pending irq
   source from the hardware block instead of irq number.

This patch together with the sh7785 specific intc tables solves
DMA controller irq issues related to buggy interrupt masking.

Reported-by: Yoshihiro Shimoda <shimoda.yoshihiro@renesas.com>
Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>