arm: mach-omap2: fdt-common: Add OP-TEE node when firmware node is defined

If a firmware node is already present in the FDT we will fail to create
one and so fail to add our OP-TEE node, make this fixup first check for
a firmware node and then only try to add one if it is not found.

Signed-off-by: Andrew F. Davis <afd@ti.com>