hwmon: (pmbus) Move pec attribute to I2C device

Enabling and disabling PEC for PMBus devices is currently only supported
with a debugfs attribute, which requires debugfs to be enabled and is
thus less than perfect. Take the lm90 driver as example and add a 'pec'
attribute to the I2C device if both the I2C adapter and the PMBus device
support it. Remove the now obsolete 'pec' attribute from debugfs.

Cc: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>