ppc4xx: Add PPC405EX support

common/serial.c

cpu/ppc4xx/4xx_enet.c

cpu/ppc4xx/Makefile

cpu/ppc4xx/cpu.c

cpu/ppc4xx/cpu_init.c

cpu/ppc4xx/interrupts.c

cpu/ppc4xx/miiphy.c

cpu/ppc4xx/ndfc.c

cpu/ppc4xx/serial.c

cpu/ppc4xx/speed.c

cpu/ppc4xx/start.S

cpu/ppc4xx/vecnum.h

include/405_mal.h

include/4xx_i2c.h

include/asm-ppc/processor.h

include/asm-ppc/u-boot.h

include/common.h

include/ppc405.h

include/ppc440.h

include/ppc4xx_enet.h