74xx_7xx/mpc86xx/ppmc7xx: Fix do_reset() declaration

The following commit:

commit 882b7d72
Author: Mike Frysinger <vapier@gentoo.org>
Date:   Wed Oct 20 03:41:17 2010 -0400

    do_reset: unify duplicate prototypes

missed the 74xx_7xx and mpc86xx arches and the ppmc7xx board do_reset()
functions which resulted in build errors such as:
  cpu.c:128: error: conflicting types for 'do_reset'
  include/command.h:102: error: previous declaration of 'do_reset' was here

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>