hwmon: (pmbus/pim4328) Add PMBus driver for PIM4006, PIM4328 and PIM4820

Add hardware monitoring support for Flex power interface modules PIM4006,
PIM4328 and PIM4820.

Signed-off-by: Erik Rosen <erik.rosen@metormote.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>