mips: dts: switch to board defines for dtb for mtmips

Previous the dts files for gardena-smart-gateway-mt7688 and
linkit-smart-7688 are set to be built when mtmips is selected.

This can lead to a compilation error if another soc is added to this arch
with different dtsi files.

So it's better to build the dtb only if their board is selected.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>