hwmon: (pmbus/core) Add lock and unlock functions

Debugfs operations may set the page number, which must be done
atomically with the subsequent i2c operation. Lock the update_lock
in the debugfs functions and provide a function for pmbus drivers
to lock and unlock the update_lock.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/20230412161526.252294-2-eajames@linux.ibm.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>