powerpc/mpc8xxx: adjust DDR burst length and chop accroding to sdram width

If the bus width is 32-bit, burst chop should be disabled and burst length
should be 8. Read from SPD or other source to determine the width.

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