net: designware: s700: Add glue code for S700 mac

This patchs adds glue logic to enable designware mac present on
Action Semi based S700 SoC, Configures SoC specific bits.

Undocumented bit that programs the PHY interface select register
comes from vendor source.

It has been tested on Cubieboard7-lite based on S700 SoC.

Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>