mpc83xx: Migrate LBLAW_* to Kconfig

The LBLAW_* values determine the window configuration of the memory
controller. Hence, they must be known at compile time, and cannot be
implemented in the DT mechanism.

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>