Add a driver for the Allwinner Reduced Serial Bus (RSB).

The RSB controller speaks a simplified two wire protocol at speeds up to
20MHz. It is used on sun8i and sun9i family SoCs to communicate with
power management ICs.

RSB isn't really I2C or SMBus, but the driver exposes an iicbus interface
to simplify power management IC drivers (which may need to support both
RSB and I2C connectivity).