ARM: dts: OMAP2+: Simplify NAND support

Commit 8c8a7771

 (ARM: OMAP2+: Add function to read GPMC settings from
device-tree) added a device-tree property "gpmc,device-nand" to indicate
is the GPMC child device is NAND. This commit should have updated the
GPMC NAND documentation (Documentation/devicetree/bindings/mtd/gpmc-nand.txt)
to list the property "gpmc,device-nand" as a required property and also
updated the example. However, this property is redundant and not needed
because the GPMC child device node for NAND is called "nand". Therefore,
remove this property.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>