docs: board: ti: Add j721s2_evm documentation

Add the documentation for J721S2-EVM and SK-AM68

TRM for J721S2/AM68: https://www.ti.com/lit/pdf/spruj28
Product Page for J721S2: https://www.ti.com/tool/J721S2XSOMXEVM
Product Page for AM68: https://www.ti.com/tool/SK-AM68



Reviewed-by: Neha Malcom Francis <n-francis@ti.com>
Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Manorit Chawdhry <m-chawdhry@ti.com>