MLK-23241 imx8dx_mek: Add iMX8DX MEK board support

MLK-23241 imx8dx_mek: Add iMX8DX MEK board support

iMX8DX MEK board has similar design with 8QXP MEK. The major changes are
1. DDR changed to 16bits 1GB DDR part
2. USB3.0 is removed and only support OTG on typec port. (No SW change needed)

This patch adds new defconfigs and DTS file for this new board.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
(cherry picked from commit 8185fa9fa8e48d64d4abf8066bf080f02343d484)
(cherry picked from commit c239a08e35459f0372d1f4b4a8366525bfdef6db)