85xx start.S cleanup and exception support

From: Ed Swarthout <Ed.Swarthout@freescale.com>

Support external interrupts from platform to eliminate system hangs.
Define CONFIG_INTERRUPTS board configure option to enable.
Enable ecm, ddr, lbc, and pci/pcie error interrupts in PIC.

Remove extra cpu initialization redundant with hardware initialization.
Whitespace cleanup.

Define and use _START_OFFSET consistent with other processors using
ppc_asm.tmpl

Move additional code from .text to boot page to make room for
exception vectors at start of image.

Handle Machine Check, External and Critical exceptions.

Fix e500 machine check error determination in traps.c

TEXT_BASE can now be 0xfffc_0000 - which cuts binary image in half.

Signed-off-by: Ed Swarthout <Ed.Swarthout@freescale.com>
Acked-by: Andy Fleming <afleming@freescale.com>