arm64: dts: rockchip: add regulator info for Kevin digitizer

We need to enable this regulator before the digitizer can be used. Wacom
recommended waiting for 100 ms before talking to the HID.

Signed-off-by: Brian Norris <briannorris@chromium.org>
[store chip ident as comment until i2c multi-compatibles are sorted]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>