stm32mp: fix name of optee reserved memory node

stm32mp: fix name of optee reserved memory node

In OP-TEE, the "optee_core@" node is reserved, appended in non secure
device tree (see mark_tzdram_as_reserved() function under CFG_DT) so
this name must be checked in optee_get_reserved_memory().
We keep the check on /reserved-memory/optee@ node to have backward
compatibility with STMT32Image booting, when the reserved node is
already present in U-Boot or SPL device tree with name "optee@".

This patch solves a boot issue on board with OP-TEE for U-Boot
compiled with stm32mp15_defconfig and without secure configuration
device tree (stm32mp157c-dk2.dts for example).

Fixes: 5fe9e0deabb1 ("stm32mp: allow calling optee_get_reserved_memory()
from U-Boot")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>