drivers: uart_xmc4xxx: Split up tx/rx into separate service requests

In preparation for async support. In async uart, service requests are
forwarded to separate dma lines. This patch splits up tx/rx into
separate service requests to enable this.

Also put service request enable code into a separate function. Before,
the same code was generated for different uart devices.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>