net: Microchip encx24j600 driver

This ethernet driver supports the Micorchip enc424j600/626j600 Ethernet
controller over a SPI bus interface. This driver makes use of the regmap API to
optimize access to registers by caching registers where possible.

Datasheet:
http://ww1.microchip.com/downloads/en/DeviceDoc/39935b.pdf

Signed-off-by: Jon Ringle <jringle@gridpoint.com>
Signed-off-by: David S. Miller <davem@davemloft.net>