dt-bindings: net: add Vertexcom MSE102x support

Add devicetree binding for the Vertexcom MSE102x Homeplug GreenPHY chip
as SPI device.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>