configs/spike_riscv64_defconfig: bump Linux kernel to 5.15.86

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/spike_riscv64_defconfig

@@ -2,7 +2,7 @@ BR2_riscv=y
 BR2_RISCV_64=y
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION=y
-BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.15.13"
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.15.86"
 BR2_LINUX_KERNEL_USE_ARCH_DEFAULT_CONFIG=y
 BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_15=y
 BR2_TARGET_ROOTFS_CPIO=y