powerpc/fsl_booke: Fix comment in head_fsl_booke.S

Fix typo in comments introduced by:

commit 6dece0eb
Author: Scott Wood <scottwood@freescale.com>
Date:   Mon Jul 25 11:29:33 2011 +0000

    powerpc/32: Pass device tree address as u64 to machine_init

Signed-off-by: Matthew McClintock <msm@freescale.com>
cc: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>