xen: dt: add dt_for_each_irq_map helper

xen: dt: add dt_for_each_irq_map helper

This function iterates over a nodes interrupt-map property and calls a
callback for each interrupt. For now it only supplies the translated
IRQ since my use case has no need of e.g. child unit address. These
can be added as needed by any future users.

This follows much the same logic as dt_irq_map_raw when parsing the
interrupt-map, but doesn't walk up the tree doing the actual
translation and it iterates over all entries instead of just looking
for the first match.

I looked into refactoring dt_irq_map_raw but I couldn't find a way
which I was confident in, plus I was reluctant to diverge from the
Linux roots of this function any further.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Julien Grall <julien.grall@citrix.com>