arm: mvebu: dts: Makefile: Compile Armada 375 dtb in a separate step

This patch changes the compilation, so that the Armada 375 board(s) are
compiled in a separate step. This is necessary for the timer dts
conversion, as A375 has a different / timer description in the dts.

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Tony Dinh <mibodhi@gmail.com>