doc: board: ti: k3: Add J784S4 EVM and AM69 SK documentation

TI K3 J784S4 and AM69 are new additions to the K3 SoC family.
Add documentation about the J784S4 EVM and AM69 SK.

Signed-off-by: Dasnavis Sabiya <sabiya.d@ti.com>
Signed-off-by: Apurva Nandan <a-nandan@ti.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com> # AM69-SK