hwmon: (pmbus/core) Protect regulator operations with mutex

In the Linux kernel, the following vulnerability has been resolved:

hwmon: (pmbus/core) Protect regulator operations with mutex

The regulator operations pmbus_regulator_get_voltage(),
pmbus_regulator_set_voltage(), and pmbus_regulator_list_voltage()
access PMBus registers and shared data but were not protected by
the update_lock mutex. This could lead to race conditions.

However, adding mutex protection directly to these functions causes
a deadlock because pmbus_regulator_notify() (which calls
regulator_notifier_call_chain()) is often called with the mutex
already held (e.g., from pmbus_fault_handler()). If a regulator
callback then calls one of the now-protected voltage functions,
it will attempt to acquire the same mutex.

Rework pmbus_regulator_notify() to utilize a worker function to
send notifications outside of the mutex protection. Events are
stored as atomics in a per-page bitmask and processed by the worker.

Initialize the worker and its associated data during regulator
registration, and ensure it is cancelled on device removal using
devm_add_action_or_reset().

While at it, remove the unnecessary include of linux/of.h.