ARM: dts: am335x: add common file for UC-2100 series

The UC-2100 series consists many boards with different peripheral
devices and wireless modules, hence we fetch common items and
create a common dtsi file to increase reusability. All boards in
UC-2100 series will include this common dtsi file.

Signed-off-by: Wes Huang (黃淵河) <wes.huang@moxa.com>
Signed-off-by: Fero JD Zhou (周俊達) <FeroJD.Zhou@moxa.com>
Signed-off-by: SZ Lin (林上智) <sz.lin@moxa.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>