arm: stm32mp: handle the OP-TEE nodes in DT with FIP support

With FIP support in TF-A (when CONFIG_STM32MP15x_STM32IMAGE
is not activated), the DT nodes needed by OP-TEE are added by OP-TEE
firmware in U-Boot device tree, present in FIP.

These nodes are only required in trusted boot, when TF-A load the file
u-boot.stm32, including the U-Boot device tree with STM32IMAGE header,
in this case OP-TEE can't update the U-Boot device tree.

Moreover in trusted boot mode with FIP, as the OP-TEE nodes are present
in U-Boot device tree only when needed the function
stm32_fdt_disable_optee can be removed.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>