mfd: rk808: Add RK818 support

The RK818 chip is a Power Management IC (PMIC) for multimedia and handheld
devices. It contains the following components:

- Regulators
- RTC
- Clocking
- Battery support

Both RK808 and RK818 chips are using a similar register map,
so we can reuse the RTC and Clocking functionality.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Tested-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>