mpc83xx: don't disable autoboot

bootdelay set to -1 'permanently' disables autobooting, even if
bootcmd is specified.  Change to a positive value to allow
autobooting when a bootcmd is set.

Reported-by: Coray Tate <Coray.Tate@freescale.com>
Cc: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>