at91: switch to Kernel 5.15 by default

Getting ready for the next release.

Claudiu said:
> I tested v5.15 on all targets I have access to previously, when
> updating OpenWrt kernel for v5.15 and when preparing this PR. (#11918)

Signed-off-by: Paul Spooren <[email protected]>
Reviewed-by: Claudiu Beznea <[email protected]>

target/linux/at91/Makefile

@@ -10,8 +10,7 @@ BOARDNAME:=Microchip (Atmel AT91)
 FEATURES:=ext4 squashfs targz usbgadget ubifs
 SUBTARGETS:=sama7 sama5 sam9x
 
-KERNEL_PATCHVER:=5.10
-KERNEL_TESTING_PATCHVER:=5.15
+KERNEL_PATCHVER:=5.15
 
 include $(INCLUDE_DIR)/target.mk