serial: Convert ARC_SERIAL to Kconfig

One step closer to completely Kconfig-driven target configuration in
U-Boot :)

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/configs/nsim_700_defconfig b/configs/nsim_700_defconfig
index 12fe5f7..01a461b 100644
--- a/configs/nsim_700_defconfig
+++ b/configs/nsim_700_defconfig
@@ -13,4 +13,5 @@
 CONFIG_OF_EMBED=y
 CONFIG_DM=y
 CONFIG_DM_SERIAL=y
+CONFIG_ARC_SERIAL=y
 CONFIG_USE_PRIVATE_LIBGCC=y