arm64: dts: rockchip: Enable mp8859 regulator on rk3399-roc-pc

The rk3399-roc-pc uses a MP8859 DC/DC converter for 12V supply.
This supplies 5V only in default state after booting.
Now we can control the output voltage via I2C interface.
Add a node for the driver to reach 12V.

Signed-off-by: Markus Reichl <m.reichl@fivetechno.de>
Link: https://lore.kernel.org/r/20200106211633.2882-6-m.reichl@fivetechno.de


Signed-off-by: Heiko Stuebner <heiko@sntech.de>