drivers: serial: serial_omap: Fix TI OMAP UART U-Boot driver to support higher baudrates

drivers: serial: serial_omap: Fix TI OMAP UART U-Boot driver to support higher baudrates

Move to OMAP specific implementation of certain ops functions as the UART
prints on the serial console fail for baudrates greater than 460800.

The MDR1 register is responsible for determining the speed mode at which
the UART should operate for OMAP specific devices. The baud divisor is used
to set the UART_DLL register which is used for generation of the baud
clock in the baud rate generator. The implementation logic is similar to
how it is implemented in omap_8250_get_divisor function of 8250_omap UART
linux driver.

Signed-off-by: Gokul Praveen <g-praveen@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>