arm64: dts: imx: Fix imx8mm-kontron-n801x-s.dtb target

While running 'make dtbs_install', the following error occurs:

make[3]: *** No rule to make target 'rootfs/freescale/imx8mm-kontron-n801x-s.dts', needed by '__dtbs_install'.

It should be .dtb, not .dts.

Fixes: 8668d8b2 ("arm64: dts: Add the Kontron i.MX8M Mini SoMs and baseboards")
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>