hwmon: pmbus: use more devres helpers

Shrink pmbus code by using devm_hwmon_device_register_with_groups()
and devm_krealloc() instead of their non-managed variants.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200824173859.4910-3-brgl@bgdev.pl
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>