Integrated OpenTitan's DPI based Virtual UART into the verilator test bench

[HU90m]

Draft because it's currently sitting atop #91

Recommend making the following change when testing:

diff --git a/sw/c/demo/hello_world/main.c b/sw/c/demo/hello_world/main.c
index 3d4b333..0ef4b4a 100644
--- a/sw/c/demo/hello_world/main.c
+++ b/sw/c/demo/hello_world/main.c
@@ -29,7 +29,7 @@ int main(void) {
 
   // This indicates how often the timer gets updated.
   timer_init();
-  timer_enable(5000000);
+  timer_enable(500);
 
   uint64_t last_elapsed_time = get_elapsed_time();

[alees24]

Some comments and suggested changes to the location and expression of the Verilator-specific top level.
Otherwise, this looks good to me, thanks.

[marnovandermaas]

Just a few comments from my end. I still need to test this PR and will approve afterwards.

[marnovandermaas]

With my initial test the uart0.log file is empty after simulation. I'll have a deeper look tomorrow.

[marnovandermaas]

Ok, both screen output and log file are now generating properly for me, so this all looks good to me!

[marnovandermaas]

Thanks so much for making these changes! This is will increase the usability of the Ibex demo system drastically 👍

[HU90m]

↑ Rebased

↓ Made suggested changes