dts/x86: use proper unit-address values

This commit changes the way some x86 devicetree set the unit-address values
of memory nodes. Before the change, they were always set to `0`. After the
change, they are derived from the `DT_DRAM_BASE` macro to match the first
address specified by the reg property.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>