bus: sunxi-rsb: Add Allwinner Reduced Serial Bus (RSB) controller bindings

Reduced Serial Bus is a proprietary 2-line push-pull serial bus supporting
multiple slave devices. It was developed by Allwinner, Inc. and used by
Allwinner and X-Powers, Inc. for their line of PMICs and other peripheral
ICs.

Recent Allwinner SoCs, starting with the A23, have an RSB controller. This
is used to talk to the PMIC, and later with the A80 and A83 platform, the
audio codec IC.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Olof Johansson <olof@lixom.net>