dt-bindings: memory-controller: Add information about ECC bindings

Add DT binding documentation for enabling ECC in the DDR sub system present
on AM64 device.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>

doc/device-tree-bindings/memory-controller/k3-j721e-ddrss.txt

@@ -13,6 +13,7 @@ Required properties:
 				  "ti,am64-ddrss" for am642
 - reg-names		cfg - Map the controller configuration region
 			ctrl_mmr_lp4 - Map LP4 register region in ctrl mmr
+			ss - Map the DDRSS configuration region
 - reg:			Contains the register map per reg-names.
 - power-domains:	Should contain two entries:
 			- an entry to TISCI DDR CFG device
@@ -32,6 +33,13 @@ Required properties:
 - ti,pi-data:		An array containing the phy independent block settings
 - ti,phy-data:		An array containing the ddr phy settings.
 
+Optional properties:
+--------------------
+- reg-names 		ss - Map the DDRSS configuration region
+- reg:			Must add "ss" to list if the above ss region is included.
+- ti,ecc-enable:	Boolean flag to enable ECC. This will reduce available DDR
+			by 1/9.
+
 Example (J721E):
 ================