ARC: HSDK-4xD: use separate config file

HSDK-4xD has quite different environment so let's split
HSDK and HSDK-4xD configs file.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/board/synopsys/hsdk/Kconfig b/board/synopsys/hsdk/Kconfig
index 5e23b32..d9c0e27 100644
--- a/board/synopsys/hsdk/Kconfig
+++ b/board/synopsys/hsdk/Kconfig
@@ -7,7 +7,8 @@
 	default "synopsys"
 
 config SYS_CONFIG_NAME
-	default "hsdk"
+	default "hsdk" if BOARD_HSDK
+	default "hsdk-4xd" if BOARD_HSDK_4XD
 
 choice
 	prompt "HSDK board type"