ARM: DT: STM32F746: add u-boot, dm-pre-reloc property to sub nodes

This patch is required for correct SPL device tree creation by fdtgrep
as fdtgrep looks for u-boot,dm-pre-reloc property of the node to include
it in the spl device tree.

Not adding it in these subnodes ignores the pin muxing of peripherals
which is almost always in the subnodes.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
Reviewed-by: Tom Rini <trini@konsulko.com>