net: mvneta: Get rid of platdata

Drop .of_to_plat() from the mvneta driver and parse the two properties
in .probe().

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>