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/board/ti/j784s4/MAINTAINERS b/board/ti/j784s4/MAINTAINERS
index 46f4ba5..b7605ff3 100644
--- a/board/ti/j784s4/MAINTAINERS
+++ b/board/ti/j784s4/MAINTAINERS
@@ -10,6 +10,7 @@
 F:	arch/arm/dts/k3-j784s4-r5-evm.dts
 F:	arch/arm/dts/k3-j784s4-ddr.dtsi
 F:	arch/arm/dts/k3-j784s4-ddr-evm-lp4-4266.dtsi
+F:	doc/board/ti/j784s4_evm.rst
 
 AM69 SK BOARD
 M:	Apurva Nandan <a-nandan@ti.com>