mips: Move cpu/mips/* to arch/mips/cpu/*

arch/mips/cpu/Makefile

arch/mips/cpu/asc_serial.c

arch/mips/cpu/asc_serial.h

arch/mips/cpu/au1x00_eth.c

arch/mips/cpu/au1x00_serial.c

arch/mips/cpu/au1x00_usb_ohci.c

arch/mips/cpu/au1x00_usb_ohci.h

arch/mips/cpu/cache.S

arch/mips/cpu/config.mk

arch/mips/cpu/cpu.c

arch/mips/cpu/incaip_clock.c

arch/mips/cpu/incaip_wdt.S

arch/mips/cpu/interrupts.c

arch/mips/cpu/start.S

board/dbau1x00/dbau1x00.c

board/gth2/gth2.c

board/pb1x00/pb1x00.c

board/purple/u-boot.lds

drivers/usb/host/isp116x-hcd.c