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/configs/stm32746g-eval_spl_defconfig b/configs/stm32746g-eval_spl_defconfig
index 55e44f8..753a4b8 100644
--- a/configs/stm32746g-eval_spl_defconfig
+++ b/configs/stm32746g-eval_spl_defconfig
@@ -69,6 +69,7 @@
 CONFIG_PHY_SMSC=y
 CONFIG_DM_ETH=y
 CONFIG_ETH_DESIGNWARE=y
+CONFIG_DW_ALTDESCRIPTOR=y
 CONFIG_MII=y
 # CONFIG_PINCTRL_FULL is not set
 CONFIG_SPL_PINCTRL=y