mtd: spi-nor: cadence-quadspi: Add TI 66AK2G SoC specific compatible

Update binding documentation to add a new compatible for TI 66AK2G SoC,
to handle TI SoC specific quirks in the driver.
Signed-off-by: Vignesh R <vigneshr@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>

Documentation/devicetree/bindings/mtd/cadence-quadspi.txt

 * Cadence Quad SPI controller
 
 Required properties:
-- compatible : Should be "cdns,qspi-nor".
+- compatible : should be one of the following:
+	Generic default - "cdns,qspi-nor".
+	For TI 66AK2G SoC - "ti,k2g-qspi", "cdns,qspi-nor".
 - reg : Contains two entries, each of which is a tuple consisting of a
 	physical address and length. The first entry is the address and
 	length of the controller register set. The second entry is the