spi_bus_lock_t struct

The core of the lock. These bits are status of the lock, which should be always available. No intermediate status is allowed. This is realized by atomic operations, mainly `atomic_fetch_and`, `atomic_fetch_or`, which atomically read the status, and bitwise write status value ORed / ANDed with given masks. The request bits together pending bits represent the actual bg request state of one device. Either one of them being active indicates the device has pending bg requests. Whenever a bit is written to the status, it means the a device on a task is trying to acquire the lock. But this will succeed only when no LOCK or BG bits active. The acquiring processor is responsible to call the scheduler to pass its role to other tasks or the BG, unless it clear the last bit in the status register.