powerpc: mpc83xx: Implement get_serial_clock()

DM serial drivers on PowerPC determine their clock frequency via the
get_serial_clock function. This function is not Implemented yet for
MPC83xx.

This patch Implements the function so that DM serial drivers work on
MPC83xx.

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