h8300: Limit timer channel ranges in Kconfig

arch/h8300/kernel/timer/itu.c and arch/h8300/kernel/timer/tpu.c
only support 0--4 for CONFIG_H8300_ITU_CH resp. H8300_TPU_CH, hence limit
them to that range in Kconfig.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>