arm64: dts: imx: Add imx8mm-prt8mm.dtb to build

imx8mm-prt8mm.dts was not getting built. Add it to the build.

Fixes: 58497d7a ("arm64: dts: imx: add Protonic PRT8MM board")
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>