sunxi: Add support for the rsb (Reduced Serial Bus)

sunxi: Add support for the rsb (Reduced Serial Bus)

sun8i (A23) introduces a new bus for communicating with the pmic, the rsb,
the rsb is also used to communicate with the pmic on the A80, and is
documented in the A80 user manual.

This commit adds support for this based on the rsb driver from the allwinner
u-boot sources.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>