arm64: dts: rockchip: add Gru/Kevin DTS

Kevin is part of a family of boards called Gru. As best as possible, the
properties shared by the Gru family are placed in rk3399-gru.dtsi, while
Kevin-specific bits are in rk3399-gru-kevin.dts. This does not add full
support for the base Gru board.

Working and tested (to some extent):
 * EC support -- including keyboard, battery, PWM, and probably more
 * UART / console
 * Thermal
 * Touchscreen
 * Touchpad
 * Digitizer (regulator still WIP)
 * PCIe / Wifi
 * Bluetooth / Webcam
 * SD card
 * eMMC
 * USB2 on TypeC
   - This works much of the time, but USB3 devices may or may not detect
     properly. Waiting on proper extcon support for USB3 over TypeC.
   - Depends on XHCI/DWC3 fixes for ARM64 that still haven't landed
 * Backlight

Not working:
 * CPUFreq -- relies on special OVP support for our PWM regulator
   circuits
 * EC / extcon support -- and with it, USB3/TypeC/DP
 * DRM -- won't even build on ARM64, so all display, eDP, etc. is not
   enabled

Not tested:
 * Audio

Signed-off-by: Brian Norris <briannorris@chromium.org>
[shared gru/kevin parts on a gru device]
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
[with a bit of reordering]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>