Make it more obvious what .dts file was expected by the build system.

When adding support for a new board, I kept getting this error message,
assuming I passed a wrong DEVICE_TREE parameter.

However, what was really the mistake was that the entry in
`arch/arm/dts/Makefile` was missing, and u-boot didn't like not being
able to find the .dtb file that should have been produced.

Simply logging the expected .dtb file name should make it easier to spot
such mistakes.

Signed-off-by: Florian Klink <flokli@flokli.de>