mpc83xx: Get rid of CONFIG_SYS_LBC_*

Except for one counter example, CONFIG_SYS_LBC_LBCR always has a value
of either 0x00040000 or 0x00000000.

CONFIG_SYS_LBC_MRTPR always has the value 0x20000000.

CONFIG_SYS_LBC_LSDMR_{1,2,4,5} are not set for any mpc83xx board.

CONFIG_SYS_LBC_LSRT is set by one board (to 0x32000000).

To simplify the configuration files, hardcode the setting of these
values for mpc83xx.

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