fdt: Add ft_system_setup() function for system device tree additions

Add an additional function for adding information to the device tree before
booting. This permits additions which are not board-specific.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Tom Rini <trini@ti.com>