sunxi: restore modified memory

Current sunxi DRAM initialisation code does several test accesses to the
DRAM array to detect aliasing effects and so determine the correct
row/column configuration. This changes the DRAM content, which breaks
use cases like soft reset and Linux's ramoops mechanism.

Fix this problem by saving and restoring the content of the DRAM cells
that is used for the test writes.

Signed-off-by: Andrey Skvortsov <andrej.skvortzov@gmail.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>