dt-bindings: mfd: Add rk806 binding

Add DT binding document for Rockchip's RK806 PMIC.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20230504173618.142075-8-sebastian.reichel@collabora.com
Signed-off-by: Lee Jones <lee@kernel.org>

Author: sre