net: dsa: vsc73xx: add support for parallel mode

This patch add platform part of vsc73xx driver.
It allows to use chip connected to a parallel memory bus and work in
memory-mapped I/O mode. (aka PI bus in chip manual)

By default device is working in big endian mode.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>