arm64: add support for i.MX8M EVK board

This is the evaluation kit board for the i.MX8M. The current level of
support yields a working console and is able to boot userspace from
SD card or Network.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com> (v1)
Reviewed-by: Rob Herring <robh@kernel.org> (v3)
Tested-by: Tested-by: Baruch Siach <baruch@tkos.co.il> (v1)
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>