Add Trim operation about gneiss

[jklmnn]

Are trim request required to be acknowledged? Since they only tell the block device that it can discard a range of blocks. Since the server is not required to provide anything to the client it shouldn't be necessary to acknowledge the trim. Aside from this the sync shouldn't be needed to acknowledged either (as it is only a marker for the server and the client can enqueue further requests after the sync and be sure that anything before the sync will be executed).
If we have to acknowledge sync and trim request the "empty" implementation on Genode would become a bit more complicated since we have to insert fake acknowledges which will also not trigger the event handler.

[senier]

Trim is just a hint to the underlying component, i don't think it has to be acknowledged.

Regarding Sync: This operation actually takes offset and block range (cf. 3.51- SYNCHRONIZE CACHE (10) of Direct Access Block commands). We should make it a "normal" request again to support those scenarios.

Regarding missing platform support: When a primitive is unsupported on a platform in general, we should not fake it, but give the client a mechanism to detect it and cope with it. Idea: Add a supported() predicate (similar to ready()) which is a precondition to Enqueue() and is implemented by the platform. Before enqueuing a request, the component has to check whether its supported and take measures if not.