microblaze: surround string default in Kconfig with double quotes

I guess this is a constant value instead of a symbol.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/arch/microblaze/Kconfig.platform b/arch/microblaze/Kconfig.platform
index f7f1739..7361974 100644
--- a/arch/microblaze/Kconfig.platform
+++ b/arch/microblaze/Kconfig.platform
@@ -65,6 +65,6 @@
 
 config XILINX_MICROBLAZE0_HW_VER
 	string "Core version number"
-	default 7.10.d
+	default "7.10.d"
 
 endmenu