[POWERPC] Update interrupt info in booting-without-of.txt

Create a new section descrbing how interrupts are represented
in the device tree.  Added more detail.  Clarified some things.

Signed-off-by: Stuart Yoder <stuart.yoder@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
1 file changed