sunxi: enable 5.15 as testing kernel

So anyone interested can help with new kernel version integration and
testing.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/sunxi/Makefile

@@ -11,6 +11,7 @@ FEATURES:=fpu usb ext4 display rootfs-part rtc squashfs
 SUBTARGETS:=cortexa8 cortexa7 cortexa53
 
 KERNEL_PATCHVER:=5.10
+KERNEL_TESTING_PATCHVER:=5.15
 
 KERNELNAME:=zImage dtbs