[MIPS] Move excite_fpga.h to include/asm-mips/mach-excite

excite_fpga.h, like all platform headers, really belongs in the
platform header directory.
Signed-off-by: NThomas Koeller <thomas.koeller@baslerweb.com>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>