net: designware: socfpga: adapt to Gen5

This driver was written for Arria10, but it applies to Gen5, too.

The main difference is that Gen5 has 2 MACs (Arria10 has 3) and the
syscon bits are encoded in the same register, thus an offset is needed.

This offset is already read from the devicetree, but for Arria10 it is
always 0, which is probably why it has been ignored. By using this
offset when writing the phy mode into the syscon regiter, we can use
this driver to set the phy mode for both of the MACs on Gen5.

Since the PHY mode bits in sysmgr are the same even for Stratix10,
let's drop the detection of the sub-mach by checking compatible
version and just use the same code for all FPGAs.

To work correctly, this driver depends on SYSCON and REGMAP, so select
those via Kconfig when it is enabeld.

Tested on socfpga_socrates (where the 2nd MAC is connected, so a shift
offset is required).

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