Convert CONFIG_CMD_GSC to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_GSC

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/gwventana_gw5904_defconfig b/configs/gwventana_gw5904_defconfig
index 49566ce..c94bedf 100644
--- a/configs/gwventana_gw5904_defconfig
+++ b/configs/gwventana_gw5904_defconfig
@@ -5,6 +5,7 @@
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
 CONFIG_TARGET_GW_VENTANA=y
 CONFIG_CMD_EECONFIG=y
+CONFIG_CMD_GSC=y
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL_WATCHDOG_SUPPORT=y