kernel: Move to Linux v5.1.8-openpower1

Powerpc related changes:

 powerpc/kexec: Fix loading of kernel + initramfs with kexec_file_load()
 powerpc/perf: Fix MMCRA corruption by bhrb_filter

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/openpower/configs/p8dtu_defconfig b/openpower/configs/p8dtu_defconfig
index d1f9279..be03f2a 100644
--- a/openpower/configs/p8dtu_defconfig
+++ b/openpower/configs/p8dtu_defconfig
@@ -17,7 +17,7 @@
 BR2_ROOTFS_POST_BUILD_SCRIPT="../openpower/scripts/fixup-target-var ../openpower/scripts/firmware-whitelist"
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION=y
-BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.1.6"
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.1.8"
 BR2_LINUX_KERNEL_PATCH="$(BR2_EXTERNAL_OP_BUILD_PATH)/linux"
 BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG=y
 BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="$(BR2_EXTERNAL_OP_BUILD_PATH)/configs/linux/skiroot_defconfig"