hwmon: (pmbus/core) Report content of CAPABILITY register in debugfs

Report the value of the CAPABILITY register in debugfs if supported.
Only check if the register is supported if PMBUS_NO_CAPABILITY
is not set.

Reviewed-by: Tzung-Bi Shih <tzungbi@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>