MIPS: xilfpga: Stop generating useless dtb.o

MIPS: xilfpga: Stop generating useless dtb.o

A dtb.o is generated from nexys4ddr.dts but this is never used since it
has been moved to mips/generic with commit b35565bb16a5 ("MIPS: generic:
Add support for MIPSfpga").

Fixes: b35565bb16a5 ("MIPS: generic: Add support for MIPSfpga")
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 4.15+
Patchwork: https://patchwork.linux-mips.org/patch/19244/
Signed-off-by: James Hogan <jhogan@kernel.org>