cc2538: uart: Make it possible to use several UARTs simultaneously

This avoids the limitation of having a single UART available at runtime, without
duplicating code.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Merge pull request #623 from ADVANSEE/cc2538-multi-uart

cc2538: uart: Make it possible to use several UARTs simultaneously