doc: board: ti: am64x: provide image alt text

Provide alternative text for image.

Fixes: 4bf49bad ("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>