lantiq: Add Linux 5.15 as testing kernel version

Signed-off-by: Martin Blumenstingl <[email protected]>

target/linux/lantiq/Makefile

@@ -10,7 +10,7 @@ FEATURES:=squashfs
 SUBTARGETS:=xrx200 xway xway_legacy falcon ase
 
 KERNEL_PATCHVER:=5.10
-KERNEL_TESTING_PATCHVER:=5.10
+KERNEL_TESTING_PATCHVER:=5.15
 
 define Target/Description
 	Build firmware images for Lantiq SoC