nios: Move cpu/nios/* to arch/nios/cpu/*

arch/nios/cpu/Makefile

arch/nios/cpu/asmi.c

arch/nios/cpu/config.mk

arch/nios/cpu/cpu.c

arch/nios/cpu/interrupts.c

arch/nios/cpu/serial.c

arch/nios/cpu/spi.c

arch/nios/cpu/start.S

arch/nios/cpu/traps.S

board/altera/dk1c20/u-boot.lds

board/altera/dk1s10/u-boot.lds

board/altera/ep1c20/u-boot.lds

board/altera/ep1s10/u-boot.lds

board/altera/ep1s40/u-boot.lds

board/psyent/pci5441/u-boot.lds

board/psyent/pk1c20/u-boot.lds

board/ssv/adnpesc1/u-boot.lds

doc/README.nios