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/drivers/net/Kconfig b/drivers/net/Kconfig
index 6126d10..9cd0d94 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -149,6 +149,12 @@
 	  This MAC is present in Intel Platform Controller Hub EG20T. It
 	  supports 10/100/1000 Mbps operation.
 
+config RGMII
+	bool "Enable RGMII"
+	help
+	  Enable the support of the Reduced Gigabit Media-Independent
+	  Interface (RGMII).
+
 config RTL8139
 	bool "Realtek 8139 series Ethernet controller driver"
 	help