dt-bindings: mtd: brcmnand: add v2.1 and v2.2 support

Add brcm,brcmnand-v2.1 and brcm,brcmnand-v2.2 as possible compatible
strings to support brcmnand controllers v2.1 and v2.2.
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200522121524.4161539-5-noltari@gmail.com

Documentation/devicetree/bindings/mtd/brcm,brcmnand.txt

@@ -20,6 +20,8 @@ Required properties:
                      "brcm,brcmnand" and an appropriate version compatibility
                      string, like "brcm,brcmnand-v7.0"
                      Possible values:
+                         brcm,brcmnand-v2.1
+                         brcm,brcmnand-v2.2
                          brcm,brcmnand-v4.0
                          brcm,brcmnand-v5.0
                          brcm,brcmnand-v6.0