dt-bindings: net: Remove gpmc-eth.txt

There is no GPMC Ethernet compatible or device driver. GPMC is
just a bus interface over which devices like Ethernet controller
can be to.

For SMSC 911x Ethernet chip bindings, please refer to
Documentation/devicetree/bindings/net/smsc,lan9115.yaml

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