board: ti: am654: Disable SA2UL node for HS devices

On HS devices the access to SA2UL is restricted on the non-secure
ARM side, disable the node in DT to prevent firewall violations.
We used to only disable the TRNG but now that we have full SA2UL
support in Linux, in which TRNG is a sub-module, disable both
by disabling the parent SA2UL node.

Signed-off-by: Andrew F. Davis <afd@ti.com>