ppc4xx: Replace 4xx lowercase SPR references

board/esd/pmc440/pmc440.c

board/mpl/mip405/mip405.c

board/mpl/pip405/pip405.c

board/netstal/hcu5/hcu5.c

board/netstal/hcu5/sdram.c

board/netstal/mcu25/mcu25.c

cpu/ppc4xx/cpu.c

cpu/ppc4xx/cpu_init.c

cpu/ppc4xx/interrupts.c

cpu/ppc4xx/speed.c

cpu/ppc4xx/start.S

include/asm-ppc/processor.h

include/ppc405.h

include/ppc440.h

post/cpu/ppc4xx/fpu.c