sencha: match sail clint frequency to cheriot-rtos board config

Cheriot-rtos' sail board config has timer_hz=2000. Match this in the
equivalent renode config so time calculations by the tickless scheduler
work as intended. Leave the sencha clint frequency as-is for now; though
it does not match the sencha.json board config because that breaks the
soundstream demo for some yet-to-be-determined reason. With this change
the cheriot-rtos testbench passes with the tickless scheduler.

Bug: 350801040

Change-Id: Ib459b9731f718e7c949673ab66d4dee7206e200d