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/stv0991_defconfig b/configs/stv0991_defconfig
index d03d984..2c14306 100644
--- a/configs/stv0991_defconfig
+++ b/configs/stv0991_defconfig
@@ -37,6 +37,7 @@
 CONFIG_PHY_MICREL_KSZ8XXX=y
 CONFIG_PHY_RESET_DELAY=10000
 CONFIG_ETH_DESIGNWARE=y
+CONFIG_DW_ALTDESCRIPTOR=y
 CONFIG_MII=y
 CONFIG_CADENCE_QSPI=y
 CONFIG_HAS_CQSPI_REF_CLK=y