arm: socfpga: gen5 enable designware_socfpga

Enable the socfpga specific designware ethernet driver by default for
socfpga by implying it when enabling CONFIG_ETH_DESIGNWARE for a
MACH_SOCFPGA config.

This is required to remove the hacky reset and phy mode handling in
arch/arm/mach-socfpga.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>