nios2: Move cpu/nios2/* to arch/nios2/cpu/*

arch/nios2/cpu/Makefile

arch/nios2/cpu/config.mk

arch/nios2/cpu/cpu.c

arch/nios2/cpu/epcs.c

arch/nios2/cpu/exceptions.S

arch/nios2/cpu/interrupts.c

arch/nios2/cpu/start.S

arch/nios2/cpu/sysid.c

arch/nios2/cpu/traps.c