fsl_ddr: Fix DDR3 calculation of rank density with 8GB or more

The calculate for rank density in compute_ranksize() for DDR3 used all
integers for the expression, so the result was also a 32-bit integer, even
though the 'bsize' variable is a u64.  Fix the expression to calculate a
true 64-bit value.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>