arm64: dts: imx: add Zii Ultra board support

The Zii Ultra design, also known as RDU3, is the i.MX8M based successor
to the the i.MX6 based RDU2. This adds the basic board support for all
components which are supported by the upstream kernel at this time.

The board comes in 2 different versions, called RMB3 and Zest, which
are derived from the same design, but have different layouts and a
few small differences in the populated components.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>