[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: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>