spl: set fdt address as spl_image arg in falcon mode

spl: set fdt address as spl_image arg in falcon mode

The arg field of `struct spl_image_info` is used by jump_to_image_linux
as the argument for the kernel in falcon mode.

Since commit 601cebc29d2a ("cmd: spl: Remove ATAG support from this
command"), fdt is the only valid argument for kernel in falcon mode.

However fdt was only being set as the argument in nor and xip boot
modes, this patch fixes it for all boot modes and removes the now
redundant code from spl_nor and spl_xip.

Signed-off-by: Anshul Dalal <anshuld@ti.com>