Bugfix for wpt/mpu backpressure from WB

Created by: silabs-oysteink

Bugfix for wpt/mpu backpressure from WB:

• resp_valid could be high for several cycles if WB was halted (debug entry or XIF). This would cause the LSU outstanding counter to underflow.
• Fixed by introducing 'resp_ready' in the LSU, which is connected to wb_ready via ready_0_i. This causes the outstanding counter to only decrement when there is a valid response and WB is ready.
• Added assertion to check that WB must be ready when a LSU response arrives from the bus.