arm64: dts: rockchip: add header for RK8XX PMIC constants

arm64: dts: rockchip: add header for RK8XX PMIC constants

To make it easier to read the device tree, let's add constants for the
rockchip,reset-mode property values that are currently only applicable
to RK806 PMIC.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
[dt-maintainers did not consider this part of the binding, so we're
 keeping the header in the devicetree directory]
Link: https://lore.kernel.org/r/20250627-rk8xx-rst-fun-v4-3-ce05d041b45f@cherry.de
Signed-off-by: Heiko Stuebner <heiko@sntech.de>