ti_armv7_common: Disable Falcon Mode on HS devices

Authentication of images in Falcon Mode is not supported. Do not enable
SPL_OS_BOOT when TI_SECURE_DEVICE is enabled. This prevents attempting
to directly load kernel images which will fail, for security reasons,
on HS devices, the board is locked if a non-authenticatable image load
is attempted, so we disable attempting Falcon Mode.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Lokesh Vutla <lokeshvutla@ti.com>