configs/spike_riscv64: bump kernel to 6.1.14

Linux Kernel 6.1 now being officially promoted to be a LTS, this
commit bump the kernel version of this defconfig to 6.1.14.

A Kernel config fragment "linux.fragment" is now needed as the kernel
no longer enable SBI v0.1 support and the earlycon RISC-V SBI in its
riscv arch defconfig. See [1] [2].

The Spike riscv-isa-sim was updated upstream accordingly [3].

In order to keep a smooth transition, this kernel config fragment
re-enable those options to make sure this kernel will work with
both old Spike versions (not including commit [3] like the v1.1.0
currently included in Buildroot), and newer versions. This commit
was also successfully tested with riscv-isa-sim at commit
0d1a48c0c0.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6f562570b9c5d6a3e30d87aec60a9d8f22a3203c
[2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3938d5a2f9369d1ebd56320629fed395ce327e9c
[3] https://github.com/riscv-software-src/riscv-isa-sim/commit/191634d2854dfed448fc323195f9b65c305e2d77

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

board/spike/riscv64/linux.fragment

@@ -0,0 +1,3 @@
+CONFIG_RISCV_SBI_V01=y
+CONFIG_HVC_RISCV_SBI=y
+CONFIG_SERIAL_EARLYCON_RISCV_SBI=y

configs/spike_riscv64_defconfig

@@ -2,9 +2,10 @@ BR2_riscv=y
 BR2_RISCV_64=y
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION=y
-BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.15.86"
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="6.1.14"
 BR2_LINUX_KERNEL_USE_ARCH_DEFAULT_CONFIG=y
-BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_5_15=y
+BR2_LINUX_KERNEL_CONFIG_FRAGMENT_FILES="board/spike/riscv64/linux.fragment"
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_6_1=y
 BR2_TARGET_ROOTFS_CPIO=y
 # BR2_TARGET_ROOTFS_TAR is not set
 BR2_TARGET_OPENSBI=y