mtd: nand: add tango NFC dt bindings doc

Add the tango NAND Flash Controller dt bindings documentation.

Signed-off-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>