dt: bindings: mtd: replace references to nand.txt with nand-controller.yaml

nand-controller.yaml replaced nand.txt however the references to it were
not updated. This change updates these references wherever it appears in
bindings documentation.

Fixes: 212e4969

 ("dt-bindings: mtd: Add YAML schemas for the generic NAND options")
Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>