MIPS: xilfpga: Actually include FDT in fitImage

Commit b35565bb ("MIPS: generic: Add support for MIPSfpga") added
and its.S file for xilfpga but forgot to add it to
arch/mips/generic/Platform so it is never used.

Fixes: b35565bb ("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/19245/


Signed-off-by: James Hogan <jhogan@kernel.org>