board: stih410-b2260: remove fdt_high and initrd_high

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: Tom Rini <trini@konsulko.com>
Reviewed-by: Patrick DELAUNAY <patrick.delaunay@st.com>