doc: Update info on using AM33xx secure devices from TI

Add a section describing the additional boot types used on AM33xx
secure devices.

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