mpc83xx: Migrate BATS config to Kconfig

The BATs (block address translation registers) determine the initial
memory window mappings. 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>