hwmon: (ltc2978) Add regulator support

Add simple on/off regulator support for ltc2978 and
other pmbus parts supported by the ltc2978 driver.

Signed-off-by: Alan Tull <atull@opensource.altera.com>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>