[POWERPC] Correct __secondary_hold comment

Remove references to pSeries and OpenFirmware in the __secondary_hold
usage comment.  __secondary_hold is a generic routine and can be used
by other platforms.
Signed-off-by: NGeoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/kernel/head_64.S

@@ -103,8 +103,8 @@ __secondary_hold_acknowledge:
 
 	. = 0x60
 /*
- * The following code is used on pSeries to hold secondary processors
- * in a spin loop after they have been freed from OpenFirmware, but
+ * The following code is used to hold secondary processors
+ * in a spin loop after they have entered the kernel, but
  * before the bulk of the kernel has been relocated.  This code
  * is relocated to physical address 0x60 before prom_init is run.
  * All of it must fit below the first exception vector at 0x100.