arm64: dts: add Rockchip rk3368 core dtsi and board dts for the r88 board

In terms of peripherals the rk3368 is quite similar to the rk3288, which
makes it possible to have a lot basic components working in the first go.
More to follow once I tracked down all the tiny differences that still
exist in some parts.

With these dts files, the R88 board is able to boot from an attached
usb device and most likely from its emmc too, if the emmc uses a standard
partition table instead of Rockchip's own one - the emmc itself is
detected correctly.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Olof Johansson <olof@lixom.net>