hwmon: (pmbus) Add debugfs for status registers

Export all the available status registers through debugfs. This is
useful for hardware diagnostics, especially on multi-page pmbus devices,
as user-space access of the i2c space could corrupt the pmbus page
accounting.

Signed-off-by: Edward A. James <eajames@us.ibm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>