To accommodate scenarios where we want to use a UART baud rate other than
the default 115,200 allow the associated compiler definition to be set
via the K3_USART_BAUD build option by updating the platform make file.

Since the platform make file now also contains the default value (still
115,200), go ahead and remove the redundant definition from the platform
header file.

Suggested-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>