ARM: dts: Add devicetree compatibles for LS1021A based boards

Add missing devicetree compatibles for the following LS1021A based
boards:

ls1021a-moxa-uc-8410a.dts
ls1021a-qds.dts
ls1021a-twr.dts

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>