spl: prevent loading args file in secure falcon mode

spl: prevent loading args file in secure falcon mode

The expected payload for the SPL in secure falcon mode is a fitImage
that contains the kernel image and the DT. This removes the need to load
an additional args file, which exposes an additional attack vector since
it can not be verified.

Therefore this patch disables loading of the arg file when
SPL_OS_BOOT_SECURE is set.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Anshul Dalal <anshuld@ti.com>