net: mvneta: Convert to use PHY_FIXED for fixed-link

Stop parsing fixed-link in the MAC driver. Instead support only PHY mode
and let the fixed PHY driver handle the fixed-link case.

Enable CONFIG_PHY_FIXED for mvneta boards that need it: Turris Omnia and
ESPRESSObin.

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