stm32mp1: spl: Update optee_get_reserved_memory() return value

In case node "/reserved-memory/optee" is not found, return -ENOENT
instead of 0.

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