MIPS: xilfpga: Removed unused header files

Header in directory asm/mach-xilfpga is not used any more.
Remove it here, and it passes to compile with xilfpga_defconfig

Signed-off-by: bibo mao <maobibo@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>