dt-bindings: mtd: remove pxa3xx NAND controller documentation

The deprecated pxa3xx_nand.c driver does not exist anymore, it has been
replaced by marvell_nand.c which has its own up-to-date documentation.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>

Documentation/devicetree/bindings/mtd/pxa3xx-nand.txt

-PXA3xx NAND DT bindings
-
-Required properties:
-
- - compatible:		Should be set to one of the following:
-			marvell,pxa3xx-nand
-			marvell,armada370-nand
-			marvell,armada-8k-nand
- - reg: 		The register base for the controller
- - interrupts:		The interrupt to map
- - #address-cells:	Set to <1> if the node includes partitions
- - marvell,system-controller: Set to retrieve the syscon node that handles
-			NAND controller related registers (only required
-			with marvell,armada-8k-nand compatible).
-
-Optional properties:
-
- - dmas:			dma data channel, see dma.txt binding doc
- - marvell,nand-enable-arbiter:	Set to enable the bus arbiter
- - marvell,nand-keep-config:	Set to keep the NAND controller config as set
-				by the bootloader
- - num-cs:			Number of chipselect lines to use
- - nand-on-flash-bbt: 		boolean to enable on flash bbt option if
-				not present false
- - nand-ecc-strength:           number of bits to correct per ECC step
- - nand-ecc-step-size:          number of data bytes covered by a single ECC step
-
-The following ECC strength and step size are currently supported:
-
- - nand-ecc-strength = <1>, nand-ecc-step-size = <512>
- - nand-ecc-strength = <4>, nand-ecc-step-size = <512>
- - nand-ecc-strength = <8>, nand-ecc-step-size = <512>
-
-Example:
-
-	nand0: nand@43100000 {
-		compatible = "marvell,pxa3xx-nand";
-		reg = <0x43100000 90>;
-		interrupts = <45>;
-		dmas = <&pdma 97 0>;
-		dma-names = "data";
-		#address-cells = <1>;
-
-		marvell,nand-enable-arbiter;
-		marvell,nand-keep-config;
-		num-cs = <1>;
-
-		/* partitions (optional) */
-	};
-