Documentation: dt: mtd: fix spelling mistake

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -52,7 +52,7 @@ Optional properties:
                               v7.0. Use this property to describe the rare
                               earlier versions of this core that include WP
 
- -- Additonal SoC-specific NAND controller properties --
+ -- Additional SoC-specific NAND controller properties --
 
 The NAND controller is integrated differently on the variety of SoCs on which it
 is found. Part of this integration involves providing status and enable bits