ARM: stm32: Use firmware property instead of loadables

There shouldn't be a need to use loadables propertyn because u-boot can be
pointed by firmware property. This change should also speedup boot process
because loadables property is list of strings which code is going through.
On the other hand firmware can just point to one image.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>