dt-bindings: mtd: Remove gpmc-nor.txt

There is no GPMC NOR compatible or device driver. GPMC is just
a bus interface over which standard (CFI/JEDC) NOR Flash chips
can be attached.

For NOR chip bindings, please refer to
Documentation/devicetree/bindings/mtd/mtd-physmap.yaml

Signed-off-by: Roger Quadros <rogerq@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>