ARM: dts: stm32: add "u-boot, dm-pre-reloc" for clk_hse in stm32f7-u-boot

In order to retrieve the clk_hse fixed clock phandle in clk_stm32f driver,
add "u-boot,dm-pre-reloc" property in Uboot specific DT file.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>