sunxi: Convert CONFIG_RGMII to Kconfig

Convert CONFIG_RGMII to Kconfig. Thanks to that, it is possible to
update defconfig files of SYS_EXTRA_OPTIONS accordingly and
remove it when it is possible.

Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
diff --git a/configs/Wits_Pro_A20_DKT_defconfig b/configs/Wits_Pro_A20_DKT_defconfig
index 97fb699..8658ef6 100644
--- a/configs/Wits_Pro_A20_DKT_defconfig
+++ b/configs/Wits_Pro_A20_DKT_defconfig
@@ -11,7 +11,6 @@
 CONFIG_DEFAULT_DEVICE_TREE="sun7i-a20-wits-pro-a20-dkt"
 CONFIG_AHCI=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
-CONFIG_SYS_EXTRA_OPTIONS="RGMII"
 CONFIG_SPL=y
 CONFIG_SPL_I2C_SUPPORT=y
 # CONFIG_CMD_IMLS is not set
@@ -21,5 +20,6 @@
 # CONFIG_SPL_ISO_PARTITION is not set
 # CONFIG_SPL_EFI_PARTITION is not set
 CONFIG_ETH_DESIGNWARE=y
+CONFIG_RGMII=y
 CONFIG_SUN7I_GMAC=y
 CONFIG_USB_EHCI_HCD=y