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
diff --git a/doc/board/ti/k3.rst b/doc/board/ti/k3.rst
index 3c33efd..15e1ed2 100644
--- a/doc/board/ti/k3.rst
+++ b/doc/board/ti/k3.rst
@@ -41,6 +41,7 @@
    ../beagle/j721e_beagleboneai64
    j721e_evm
    j721s2_evm
+   j784s4_evm
 
 Boot Flow Overview
 ------------------