layerscape: switch kernel testing to 5.15

Let's test layerscape with 5.15 kernel.

Run tested: LS1012A-FRDM, LS1046A-RDB (nor and sd-boot images),
	    LS1021A-IOT (By @ArtelMike with u-boot fix)

Signed-off-by: Pawel Dembicki <[email protected]>

target/linux/layerscape/Makefile

@@ -7,7 +7,7 @@ include $(TOPDIR)/rules.mk
 BOARD:=layerscape
 BOARDNAME:=NXP Layerscape
 KERNEL_PATCHVER:=5.10
-KERNEL_TESTING_PATCHVER:=5.10
+KERNEL_TESTING_PATCHVER:=5.15
 FEATURES:=squashfs nand usb pcie gpio fpu ubifs ext4 rootfs-part boot-part
 SUBTARGETS:=armv8_64b armv7