arm: spl: prepare for jumping to OPTEE

Make sure to (if applicable) flush the D-cache, invalidate I-cache,
and disable MMU and caches before jumping to OPTEE.
This fixes the SDP->SPL->OPTEE boot flow on iMX6Q and most likely on
some other ARM SoCs.

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>
Co-developed-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>