layerscape: switch kernel to 6.1 version

It was testing since couple months. Lets switch it to 6.1 by default for
wide testing.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>

target/linux/layerscape/Makefile

@@ -7,8 +7,7 @@ include $(TOPDIR)/rules.mk
 BOARD:=layerscape
 BOARDNAME:=NXP Layerscape
 
-KERNEL_PATCHVER:=5.15
-KERNEL_TESTING_PATCHVER:=6.1
+KERNEL_PATCHVER:=6.1
 
 FEATURES:=squashfs nand usb pcie gpio fpu ubifs ext4 rootfs-part boot-part
 SUBTARGETS:=armv8_64b armv7