arm: socfpga: gen5: remove hacked ETH RST handling

The 'dwmac_socfpga' ETH driver can now get the MACs out of reset
via the socfpga reset driver and can set PHY mode via syscon.

This means we can now remove the ad-hoc code to do this from
arch/arm/mach-socfpga.

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