[POWERPC] Move of_irq_to_resource from prom.h to prom_parse.c

In the powerpc architecture, of_irq_to_resource, currently sitting in
prom.h, needs irq_of_parse_and_map and NO_IRQ from asm-powerpc/irq.h.
The solution suggested by Benjamin Herrenschmidt is to move it to
arch/powerpc/kernel/prom_parse.c.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>