optee: fix copy of optee reserved-memory node

Fix the loop that parses FDT for a reserved memory node named "optee".

Before this change, if at least one subnode was found in the
reserved-memory node, the function endlessly looped since instruction
continue returned back in the loop without updating variable subnode.
This change fixes the issue by using a for loop.

Fixes: 6ccb05ea ("image: fdt: copy possible optee nodes to a loaded devicetree")
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>