board: stm32: remove fdt_high and fdt_highinitrd_high

For stm32 f4, f7 and h7 boards, remove fdt_high and
initrd_high as they shouldn't be used, this allows the fdt
and initrd relocation.
This implies to set CONFIG_SYS_BOOTMAPSZ to indicate the
amount of memory available to contain kernel, device tree
and initrd for relocation.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick DELAUNAY <patrick.delaunay@st.com>