microblaze: Add missing quotes around XILINX_MICROBLAZE0_HW_VER

This issue is reported by kconfiglib:
warning: style: quotes recommended around default value for string
symbol XILINX_MICROBLAZE0_HW_VER (defined at
board/xilinx/microblaze-generic/Kconfig:37)

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/board/xilinx/microblaze-generic/Kconfig b/board/xilinx/microblaze-generic/Kconfig
index 6e93a3d..f2fa0f7 100644
--- a/board/xilinx/microblaze-generic/Kconfig
+++ b/board/xilinx/microblaze-generic/Kconfig
@@ -36,6 +36,6 @@
 
 config XILINX_MICROBLAZE0_HW_VER
 	string "Core version number"
-	default 7.10.d
+	default "7.10.d"
 
 endif