powerpc/mpc8xxx: reword max tCKmin message

Reword "The DIMM max tCKmin is ..." to "The DDR clock is faster than the slowest
DIMM(s) can support". Fixed interger type in printf as well.

Signed-off-by: York Sun <yorksun@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>