sh: Add support for SH7721 CPU subtype.

arch/sh/Kconfig

arch/sh/Kconfig.debug

arch/sh/drivers/dma/Kconfig

arch/sh/drivers/dma/dma-sh.c

arch/sh/kernel/cpu/sh3/Makefile

arch/sh/kernel/cpu/sh3/probe.c

arch/sh/kernel/cpu/sh3/setup-sh7720.c

arch/sh/kernel/early_printk.c

arch/sh/kernel/setup.c

arch/sh/kernel/timers/timer-tmu.c

drivers/serial/sh-sci.c

drivers/serial/sh-sci.h

include/asm-sh/cpu-sh3/cache.h

include/asm-sh/cpu-sh3/dma.h

include/asm-sh/cpu-sh3/gpio.h

include/asm-sh/cpu-sh3/mmu_context.h

include/asm-sh/cpu-sh3/timer.h

include/asm-sh/cpu-sh3/ubc.h

include/asm-sh/processor.h