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>

arch/mips/generic/Platform

@@ -16,3 +16,4 @@ all-$(CONFIG_MIPS_GENERIC)	:= vmlinux.gz.itb
 its-y					:= vmlinux.its.S
 its-$(CONFIG_FIT_IMAGE_FDT_BOSTON)	+= board-boston.its.S
 its-$(CONFIG_FIT_IMAGE_FDT_NI169445)	+= board-ni169445.its.S
+its-$(CONFIG_FIT_IMAGE_FDT_XILFPGA)	+= board-xilfpga.its.S