mb/lowrisc/nexys4ddr: Actually fix the UART clock setup

mb/lowrisc/nexys4ddr: Actually fix the UART clock setup

Ron's code calculated the DLL and DLM registers of the 8250 UART, but
that's the job of the UART driver. uart_input_clock_divider isn't needed
anymore because the default value of 16 works.

As a bonus, the baud rate can now be selected in Kconfig, instead of
being hardcoded at 115200.

TEST=Booted the board at 9600 and 115200 baud.

Change-Id: I3d5e49568b798a6a6d944db1161def7d0a2d3b48
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/17188
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>