mfd: RK808: Add new mfd driver for RK808

The RK808 chip is a power management IC for multimedia and handheld
devices. It contains the following components:

- Regulators
- RTC
- Clkout

The RK808 core driver is registered as a platform driver and provides
communication through I2C with the host device for the different
components.

Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Zhang Qing <zhangqing@rock-chips.com>
Tested-by: Heiko <heiko@sntech.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>