cpm_uart: Support uart_wait_until_sent()

Set port->fifosize to the software FIFO size, and update the port timeout
when the baud rate is modified. SCC ports have an optional 32 byte hardware
FIFO which is currently not taken into account, as there is no documented way
to check when the FIFO becomes empty.

Signed-off-by: Laurent Pinchart <laurentp@cse-semaphore.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>