ARM: dts: socfpga: Add unit name to memory nodes

Memory nodes in Arria5, Cyclone5 and Arria10 do not have a unit name.
This will trigger several warnings like this one (when compiled with
W=1):

Node /memory has a reg or ranges property, but no unit name

Add the corresponding unit name to each node.

Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>