Integrate uart dpi testbench in GLIP

[imphil]

At https://github.com/wallento/uartdpi Stefan developed an UART DPI implementation, which essentially simulates a UART interface.

Subtasks:

• Create a new demo in the GLIP tree (at backend_uart/logic/demo/uartdpi most likely)
• Copy the uartdpi implementation there
• Create a toplevel testbench which integrates the glip uart toplevel and the uartdpi implementation.
• Create a Makefile or other build scripts to run the testbench in simulation (using ModelSim?)
• Document how to build and run the loopback test against this testbench.

[imphil]

@koenenwmn is this something you could have a look at?