Provide alternative text for image.
Fixes: 4bf49bade1 ("doc: board: ti: am64: Add boot flow diagram")
Reported-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>