[POWERPC] New device-tree interrupt parsing code

Adds new routines to prom_parse to walk the device-tree for interrupt
information. This includes both direct mapping of interrupts and low
level parsing functions for use with partial trees.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>