nios2: fix building all dtbs

nios2 has a 'dtbs' target, but nothing is added to 'dtb-*' targets and
no dtbs were getting built. This enables building all the dts files in
arch/nios2/boot/dts/ when COMPILE_TEST and OF_ALL_DTBS are enabled.

Cc: Ley Foon Tan <lftan@altera.com>
Cc: nios2-dev@lists.rocketboards.org
Signed-off-by: Rob Herring <robh@kernel.org>