imx8m: soc: cope with existing optee node

imx8m: soc: cope with existing optee node

On i.MX8M SoCs, the /firmware/optee Devicetree node is created just before
booting the OS when OP-TEE is found running. If the node already exists,
this results in an error, which prevents the OS to boot:

  Could not create optee node.
  ERROR: system-specific fdt fixup failed: FDT_ERR_EXISTS
   - must RESET the board to recover.

  failed to process device tree

On the i.MX8M systems where CONFIG_OF_SYSTEM_SETUP is defined, the
ft_add_optee_node() function is called before booting the OS. It will
create the OP-TEE Devicetree node and populate it with reserved memory
informations gathered at runtime.

On on most i.MX8M systems the Devicetree is built with an optee node if
CONFIG_OPTEE is defined. This node is indeed necessary for commands and
drivers communicating with OP-TEE, even before attempting OS boot.

The aforementioned issue can happen on the Compulab IOT-GATE-iMX8, which is
the only in-tree i.MX8M system where both CONFIG_OPTEE and
CONFIG_OF_SYSTEM_SETUP are defined (see the imx8mm-cl-iot-gate*
defconfigs).

Deal with an existing optee node gracefully at runtime to fix this issue.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Tim Harvey <tharvey@gateworks.com>