net: gem: Move driver to DM

net: gem: Move driver to DM

- Enable DM_ETH by default for Zynq and ZynqMP
- Remove board_eth_init code
- Change miiphy_read function to return value instead of error code
  based on DM requirement
- Do not enable EMIO DT support by default

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>