i386: Move cpu/i386/* to arch/i386/cpu/*

arch/i386/cpu/Makefile

arch/i386/cpu/config.mk

arch/i386/cpu/cpu.c

arch/i386/cpu/interrupts.c

arch/i386/cpu/resetvec.S

arch/i386/cpu/sc520/Makefile

arch/i386/cpu/sc520/sc520.c

arch/i386/cpu/sc520/sc520_asm.S

arch/i386/cpu/sc520/sc520_pci.c

arch/i386/cpu/sc520/sc520_ssi.c

arch/i386/cpu/sc520/sc520_timer.c

arch/i386/cpu/serial.c

arch/i386/cpu/start.S

arch/i386/cpu/start16.S

arch/i386/include/asm/interrupt.h

arch/i386/include/asm/u-boot-i386.h

doc/TODO-i386