sparc32: introduce build_device_irq

build_device_irq() is used to encapsulate the plaform
specific details when we build an irq.
For now the default is a simple 1:1 but sun4d differs.
This patch refactors functionality - but does not change
the existing functionality.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>