CPSW driver has multiple compatibles for errata implementations but not
documented, add necessary documentation.
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Rob Herring <robh@kernel.org>
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 Required properties:
-- compatible           : Should be "ti,cpsw"
+- compatible           : Should be one of the below:-
+                         "ti,cpsw" for backward compatible
+                         "ti,am335x-cpsw" for AM335x controllers
+                         "ti,am4372-cpsw" for AM437x controllers
+                         "ti,dra7-cpsw" for DRA7x controllers
 - reg                  : physical base address and size of the cpsw
                          registers map
 - interrupts           : property with a value describing the interrupt