Add Urukul proto_rev failure

Signed-off-by: Egor Savkin <es@m-labs.hk>

src/hw/urukul.md

@@ -95,3 +95,11 @@ It may be due to misconfiguration of SUServo. Check that both firmware and pins
 
 This can happen due to lack/bad clock source connection. Check that clock source is connected respective to the customer needs,
 and if it is connected to the [Clocker](clocker.md), check that clocker receives clock signal properly.
+
+### Urukul proto_rev mismatch
+
+```pycon
+ValueError: Urukul proto_rev mismatch
+```
+
+Check the ports are connected respectively to the JSON description.