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>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 408e4ff..67d4fd5 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -551,6 +551,7 @@
 	select OF_CONTROL
 	select SPL_OF_CONTROL
 	select DM
+	select DM_ETH
 	select SPL_DM
 	select DM_SPI
 	select DM_SERIAL
@@ -562,6 +563,7 @@
 	select ARM64
 	select DM
 	select OF_CONTROL
+	select DM_ETH
 	select DM_SERIAL
 
 config TEGRA