arm: mx6: ddr: fix cs0_end calculation

Current way of calculation CS0_END field for MMDCx_MDASP register
is problematic because in most cases the user is forced to define
cs_density in an unnatural way: as value - 2, instead of value.

This breaks the abstraction provided by struct mx6_ddr_sysinfo
because the user is forced to be aware of the way the calculation
is performed.

Refactor the calculation.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>