spl: make args file optional in falcon mode

spl: make args file optional in falcon mode

Falcon mode loads a kernel file and an args file which is the
device-tree. However in the case of kernel file being a FIT that
contains the device-tree within it, loading the args file is not
required.

Therefore, this patch introduces a new SPL_OS_BOOT_ARGS config options
that allows us to enable or disable loading of the args file in falcon
mode.

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