ddr: marvell: a38x: fix memory cs size function

commit c8b301463d508c807a33f7b7eaea98bbda4aa35e upstream.

The funtion returnd cs size in byte instead of MB, that cause
calculation error since the caller was expected to get u32 and when he
got above 4G it refers it as 0.
The fix was to get the cs memory size from function as in MB and then
multiply it by 1MB.

Signed-off-by: Moti Buskila <motib@marvell.com>
Reviewed-by: Kostya Porotchkin <kostap@marvell.com>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz>