MIPS: Xilfpga: Switch to using generic defconfigs

Use the generic platform code and remove arch/mips/xilfpga

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15847/


[jhogan@kernel.org: Update arch/mips/Makefile snippet and move to end]
Signed-off-by: James Hogan <jhogan@kernel.org>