Add workaround for CESA MBUS windows with 4GB DRAM

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 (it has to be power-of-2).

Submitted by: Marcin Wojtas <mw@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential revision: https://reviews.freebsd.org/D10724