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>

include/netdev.h

@@ -87,8 +87,6 @@ int xilinx_emaclite_initialize(bd_t *bis, unsigned long base_addr,
 							int txpp, int rxpp);
 int xilinx_ll_temac_eth_init(bd_t *bis, unsigned long base_addr, int flags,
 						unsigned long ctrl_addr);
-int zynq_gem_initialize(bd_t *bis, phys_addr_t base_addr,
-			int phy_addr, u32 emio);
 /*
  * As long as the Xilinx xps_ll_temac ethernet driver has not its own interface
  * exported by a public hader file, we need a global definition at this point.