Kconfig: comply to Linux 5.3's Kconfig language rules

Kconfig became stricter on what it accepts, so accomodate before
updating to a new release.

Change-Id: I92a9e9bf0d557a7532ba533cd7776c48f2488f91
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/37156
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/southbridge/amd/agesa/Kconfig

@@ -13,4 +13,4 @@
 # GNU General Public License for more details.
 #
 
-source src/southbridge/amd/agesa/hudson/Kconfig
+source "src/southbridge/amd/agesa/hudson/Kconfig"

src/southbridge/amd/cimx/Kconfig

@@ -17,4 +17,4 @@ config AMD_SB_CIMX
 	bool
 	default n
 
-source src/southbridge/amd/cimx/sb800/Kconfig
+source "src/southbridge/amd/cimx/sb800/Kconfig"

src/southbridge/amd/pi/Kconfig

@@ -13,4 +13,4 @@
 # GNU General Public License for more details.
 #
 
-source src/southbridge/amd/pi/hudson/Kconfig
+source "src/southbridge/amd/pi/hudson/Kconfig"