gemini: Bump kernel to v5.15

It's working well on all tested targets, so let's move
Gemini forward to v5.15. imx is already bumped so why not.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

target/linux/gemini/Makefile

@@ -10,7 +10,7 @@ BOARDNAME:=Cortina Systems CS351x
 FEATURES:=squashfs pci rtc usb dt gpio display ext4 rootfs-part boot-part
 CPU_TYPE:=fa526
 
-KERNEL_PATCHVER:=5.10
+KERNEL_PATCHVER:=5.15
 
 define Target/Description
 	Build firmware images for the StorLink/Cortina Gemini CS351x ARM FA526 CPU