Add workaround for CESA MBUS windows with 4GB DRAM

[mw_semihalf.com]

Summary

Armada 38x SoC's equipped with 4GB DRAM suffer freeze
during CESA operation, if MBUS window opened at given
DRAM CS reaches end of the address space. Apply a workaround
by setting the window size to the closest possible
value, i.e. divide it by 2.