arm: mx6: ddr: fix cs0_end calculation

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 <[email protected]>
Cc: Tim Harvey <[email protected]>
Signed-off-by: Nikita Kiryanov <[email protected]>