imx: 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/imx/Makefile

@@ -11,6 +11,7 @@ FEATURES:=audio display fpu gpio pcie rtc usb usbgadget squashfs targz nand ubif
 SUBTARGETS:=cortexa7 cortexa9
 
 KERNEL_PATCHVER:=5.10
+KERNEL_TESTING_PATCHVER:=5.15
 
 include $(INCLUDE_DIR)/target.mk