drivers: Add Atmel SAM family GMAC Ethernet driver

This is a zero-copy networking implementation of Ethernet driver.

Limitations:
- one shot PHY setup, no support for PHY disconnect/reconnect
- no support for devices with DCache enabled due to missing
  non-cacheable RAM regions in Zephyr.

Tested on Atmel SMART SAM E70 Xplained board

Origin: Original

Jira: ZEP-1492
Change-Id: Ib944f91193efbd12c1142b0bcf1f635388bf1b87
Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>