imx6: Remove unneeded CONFIG_DM_MDIO

As explained in the CONFIG_DM_MDIO text inside drivers/net/Kconfig:

"Useful in particular for systems that support
DM_ETH and have a stand-alone MDIO hardware block shared by multiple
Ethernet interfaces."

i.MX6 has a single FEC instance, so there is no need to select
CONFIG_DM_MDIO.

Remove it from the i.MX6 defconfig files.
Signed-off-by: NFabio Estevam <festevam@gmail.com>

configs/apalis_imx6_defconfig

@@ -74,7 +74,6 @@ CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ90X1=y
 CONFIG_DM_ETH=y
-CONFIG_DM_MDIO=y
 CONFIG_FEC_MXC=y
 CONFIG_MII=y
 CONFIG_PINCTRL=y

configs/colibri_imx6_defconfig

@@ -73,7 +73,6 @@ CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ8XXX=y
 CONFIG_DM_ETH=y
-CONFIG_DM_MDIO=y
 CONFIG_FEC_MXC=y
 CONFIG_MII=y
 CONFIG_PINCTRL=y

configs/dh_imx6_defconfig

@@ -77,7 +77,6 @@ CONFIG_PHYLIB=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ90X1=y
 CONFIG_DM_ETH=y
-CONFIG_DM_MDIO=y
 CONFIG_FEC_MXC=y
 CONFIG_MII=y
 CONFIG_PINCTRL=y

configs/mx6sabreauto_defconfig

@@ -76,7 +76,6 @@ CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_ATHEROS=y
 CONFIG_DM_ETH=y
-CONFIG_DM_MDIO=y
 CONFIG_FEC_MXC=y
 CONFIG_RGMII=y
 CONFIG_MII=y

configs/mx6sabresd_defconfig

@@ -83,7 +83,6 @@ CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_PHYLIB=y
 CONFIG_PHY_ATHEROS=y
 CONFIG_DM_ETH=y
-CONFIG_DM_MDIO=y
 CONFIG_FEC_MXC=y
 CONFIG_RGMII=y
 CONFIG_MII=y