[POWERPC] start-cpu has no return code

[POWERPC] start-cpu has no return code

According to this document:

 PowerPC Processor binding to: IEEE 1275-1994
 Standard for Boot (Initialization, Configuration) Firmware
 Revision: 2.1 (Approved Version)
 Date: November 6, 1996

The start-cpu client interface has the following signature:

 start-cpu
  IN: nodeid, pc, arg
  OUT: none

This patch reflects that, and makes us stop seeing a bogus OF_FAILURE if
we do actually check the return code of of_start_cpu.  Tested on JS20
and JS21 blades.

Signed-off-by: Amos Waterland <apw@us.ibm.com>
Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>