stm32mp: Increase the reserved memory in board_get_usable_ram_top

Add 8M for the U-Boot reserved memory (display, fdt, gd, ...)
mapped cacheable before relocation.

Without this patch the device tree, located before the MALLOC area
is not tagged cacheable just after relocation, before mmu reconfiguration.

This patch reduces the duration for device tree parsing in
lmb_init_and_reserve.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>