m68k: mcf523x: move CPU type to Kconfig and refactor config.mk

Move the CPU type config options from include/configs/M5235EVB.h
to arch/m68k/Kconfig and refactor the CPU flags select in
arch/m68k/cpu/mcf523x/config.mk.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Alison Wang <alison.wang@freescale.com>
Cc: Angelo Dureghello <angelo@sysam.it>