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>

arch/microblaze/Kconfig.platform

@@ -65,6 +65,6 @@ config XILINX_MICROBLAZE0_USE_FPU
 
 config XILINX_MICROBLAZE0_HW_VER
 	string "Core version number"
-	default 7.10.d
+	default "7.10.d"
 
 endmenu