fsl-ddr: Fix the chip-select interleaving issue

commit 1542fbde
introduced one new bug to chip-select interleaving.

Single DDR controller also can do the chip-select
interleaving if there is dual-rank or qual-rank DIMMs.

Signed-off-by: Dave Liu <daveliu@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>