board: freescale: t104xrdb: implement get_serial_clock

The serial clock is provided by the get_serial_clock() callback on PPC
under DM_SERIAL. Use the same method to compute the clock as for
non-DM_SERIAL use cases.

Signed-off-by: Camelia Groza <camelia.groza@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>