Makefile: support building SPL FIT images without device trees

When building a U-Boot FIT image, the device trees specified by the
board are unconditionally built for inclusion in the FIT image. However,
not all device tree providers, such as CONFIG_OF_PRIOR_STAGE, require a
device tree to be built and bundled with the U-Boot binary. They rely on
other mechanisms to provide the device tree to U-Boot. Compilation on
boards with these device tree providers fails, because they do not
specify a device tree.

Change the makefile rules to conditionally build the device trees if
CONFIG_OF_SEPARATE, CONFIG_OF_EMBED, or CONFIG_OF_HOSTFILE is selected
as device tree provider.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>