drivers: ethernet: Add support for KSZ8794 DSA device

This patch adds support for Microchip's KSZ8794 DSA device, which for
switch and PHY control uses SPI communication.

This driver also provides support for handling tail tagging added and
decoded in the KSZ8794 device as well as modifying entries in the
static MAC address table.

It is also possible to use GPIO pin to reset KSZ8794 switch.
When the "reset-gpios" property is not defined, software based reset
is performed instead.

The KSZ8794 can now be used on boards which have SPI CS only available
as GPIO pin.

Signed-off-by: Stefan Bigler <stefan.bigler@securiton.ch>
Signed-off-by: Lukasz Majewski <lukma@denx.de>