dts/Makefile: Add build support for LS2080a QDS & RDB board DTS

This patch adds build support for LS2080a QDS & RDB board DTS files
in the arm64 DTS Makefile.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@freescale.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>