powerpc/85xx: issue fsl_soc reboot warning only when applicable

Some CPU, like the MPC8560 don't have a RSTCR in the Global
Utilities Block.  These boards will implement their own reboot
call, and not use this code, so we should only warn about the
absence of the GUTS RSTCR when the default reboot code is used.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>