icebreaker: fix UART flow control pins.

[alanvgreen]

Adds a role attribute to the ICEBreakerPlatform's UARTResource.

Since commit 5d56f7f all uses of
UARTResource must specify a role attribute. Without this role attribute,
icebreaker.py cannot be imported.

[alanvgreen]

I chose "dce" as the value for the role attribute. I'm not entirely sure this is right. However, I figured it was better to have a slightly wrong but importable ICEBreakerPlatform than to have an unimportable ICEBreakerPlatform.

[whitequark]

There's no control flow on icebreaker, so the role is irrelevant.

[whitequark]

Fixed in a889afe.