driver: eth: Implementation of Open Alliance's TC6 T1S communication

Those files provide generic functions to handle transmission between chip
conforming OA TC6 standard and Zephyr's network stack represented by
struct net_pkt.

The communication is performed via SPI and is focused on reduced memory
footprint (works with SOC equipped with 32 KiB of RAM) and robustness of
operation.

Signed-off-by: Lukasz Majewski <lukma@denx.de>