mfd: Add Gateworks System Controller core driver

The Gateworks System Controller (GSC) is an I2C slave controller
implemented with an MSP430 micro-controller whose firmware embeds the
following features:
- I/O expander (16 GPIO's) using PCA955x protocol
- Real Time Clock using DS1672 protocol
- User EEPROM using AT24 protocol
- HWMON using custom protocol
- Interrupt controller with tamper detect, user pushbotton
- Watchdog controller capable of full board power-cycle
- Power Control capable of full board power-cycle

see http://trac.gateworks.com/wiki/gsc for more details

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>