board: stm32mp1: fix handling of DT OP-TEE reserved memory nodes

Fix the sequence in stm32mp1 fdt.c that disables OP-TEE resources
defined in FDT when U-boot detects OP-TEE firmware is not present.

Before this change, helper function stm32_fdt_disable_optee()
set property status to "disabled" for the OP-TEE reserved memory
nodes but this has no impact since Linux kernel does not consider
the status property for reserved-memory subnodes. This change
make U-Boot to attempt to delete the node instead.

Fixes: 4a1b975d ("board: stm32mp1: reserve memory for OP-TEE in device tree")
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>