mediatek: expose testing Linux 6.1 testing kernel

Set KERNEL_TESTING_PATCHVER:=6.1 to allow building all subtargets with
Linux 6.1.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

target/linux/mediatek/Makefile

@@ -9,6 +9,7 @@ SUBTARGETS:=mt7622 mt7623 mt7629 filogic
 FEATURES:=dt-overlay emmc fpu gpio nand pci pcie rootfs-part separate_ramdisk squashfs usb
 
 KERNEL_PATCHVER:=5.15
+KERNEL_TESTING_PATCHVER:=6.1
 
 include $(INCLUDE_DIR)/target.mk
 DEFAULT_PACKAGES += \