doc: board: ti: k3: Mention TI_DM argument

Mention TI_DM argument can be used to fetch a custom DM binary in the
A72 build instructions for K3 devices.

Signed-off-by: Neha Malcom Francis <n-francis@ti.com>
Reviewed-by: Andrew Davis <afd@ti.com>
Reviewed-by: Manorit Chawdhry <m-chawdhry@ti.com>