net: ethernet: dwmac: add ethernet glue logic for NXP imx8 chip

NXP imx8 family like imx8mp/imx8dxl chips support Synopsys MAC 5.10a IP.
This patch adds settings for NXP imx8 glue layer:
- clocks
- dwmac address width
- phy interface mode selection
- adjust rgmii txclk rate

v2:
- adjust code sequences in order to have reverse christmas
  tree local variable ordering.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>