[POWERPC] Rewrite the PPC 86xx IRQ handling to use Flat Device Tree

IRQ setup now comes from the Flat Device Tree and use the new generic
IRQ code.  Fixed the fsl_soc.c IRQ OF interrupt node parsing.
Removed some unused MPC86xx macro definition.

Signed-off-by: Zhang Wei <wei.zhang@freescale.com>
Signed-off-by: Jon Loeliger <jdl@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>
(cherry picked from 919fede6 commit)