spl: fit: use U-Boot device tree when FIT image has no device tree

spl: fit: use U-Boot device tree when FIT image has no device tree

As part of the SPL FIT boot flow, the device tree is appended to U-Boot
proper. The device tree is used to record information on the loadables
to make them available to the SPL framework and U-Boot proper. Depending
on the U-Boot device tree provider, the FIT image might not include a
device tree. Information on the loadables is missing in this case.

When booting via firmware bundled with the FIT image, U-Boot SPL loads
the firmware binary and U-Boot proper before starting the firmware. The
firmware, in turn, is responsible for starting U-Boot proper.
Information on the memory location of the U-Boot proper loadable must be
available to the SPL framework so that it can be passed to the firmware
binary. To support this use case when no device tree is found in the FIT
image, fall back to the U-Boot device tree in this situation.

At the same time, update the comment to remove the note that the
destination address must be aligned to ARCH_DMA_MINALIGN. Alignment is
only required as an intermediate step when reading external data. This
is automatically handled by spl_fit_append_fdt(). After reading the
external data, it is copied to the specified address, which does not
have to be aligned to ARCH_DMA_MINALIGN.

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>