mb/amd/mandolin: Quote string in Kconfig that contains /

Newer versions of Kconfig require that.

Change-Id: I95f889d462ace1b912b5e6c7320973e8a826f3cb
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42773
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/amd/mandolin/Kconfig

@@ -31,7 +31,7 @@ config CBFS_SIZE
 
 config MAINBOARD_DIR
 	string
-	default amd/mandolin
+	default "amd/mandolin"
 
 config MAINBOARD_PART_NUMBER
 	string