UART4 is used as CM7 coprocessor debug UART and may not be accessible from
Linux in case it is protected by RDC. The RDC protection is set up by the
platform firmware. UART4 is not used on this platform by Linux. Disable
UART4 by default to prevent boot hangs, which occur when the RDC protection
is in place.

Fixes: 562d222f ("arm64: dts: imx8mp: Add support for Data Modul i.MX8M Plus eDM SBC")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>