mpc83xx: Kconfig: Migrate HRCW to Kconfig

The HRCW (hardware reset configuration word) is a constant that must be
hard-coded into the boot loader image. So, it must be available at
compile time, and cannot be migrated to the DT mechanism, but has to be
kept in Kconfig.

Configuration of this crucial variable should still be somewhat
comfortable. Hence, make its fields configurable in Kconfig, and
assemble the final value from these.

Signed-off-by: Mario Six <mario.six@gdsys.cc>