Rework locking of processor-internal bus

[stnolting]

With this PR, bus locking is implemented for the entire bus infrastructure. When the bus is locked, the locking host has exclusive bus access (no interleaving) until it releases the lock.

1. The bus is locked by a host by asserting lock together with stb.
2. lock is set until the end of the locked bus access(es).
3. The bus is locked when the host received the (first) ack.
4. The host can issue further transfer requests (stb) while the bus is locked.
5. The lock is released when the host clears lock.