Convert CONFIG_DW_ALTDESCRIPTOR to Kconfig

This converts the following to Kconfig:
   CONFIG_DW_ALTDESCRIPTOR

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index 56f9416..40b5c82 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -312,6 +312,7 @@
 config ETH_DESIGNWARE_SOCFPGA
 	select REGMAP
 	select SYSCON
+	select DW_ALTDESCRIPTOR
 	bool "Altera SoCFPGA extras for Synopsys Designware Ethernet MAC"
 	depends on DM_ETH && ETH_DESIGNWARE
 	help
@@ -326,6 +327,10 @@
 	  This provides glue layer to use Synopsys Designware Ethernet MAC
 	  present on Actions S700 SoC.
 
+config DW_ALTDESCRIPTOR
+	bool "Designware Ethernet MAC uses alternate (enhanced) descriptors"
+	depends on ETH_DESIGNWARE
+
 config ETHOC
 	bool "OpenCores 10/100 Mbps Ethernet MAC"
 	help