MIPS: move au1x00 SoC code to arch/mips/mach-au1x00

Move all au1x00 code out of arch/mips/cpu/mips32 to allow
unification of CPU code in a later patch. The reorganization
of the SoC specific header files will be done in a later patch
series.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Paul Burton <paul.burton@imgtec.com>