kernel: Move to Linux v5.1.15-openpower1

Powerpc related changes:

 powerpc/mm/64s/hash: Reallocate context ids on fork
 powerpc/bpf: use unsigned division instruction for 64-bit operations
 powerpc/powernv: Return for invalid IMC domain
 powerpc/64s: Fix THP PMD collapse serialisation

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/openpower/configs/pseries_defconfig b/openpower/configs/pseries_defconfig
index c513f4b..5a75bc4 100644
--- a/openpower/configs/pseries_defconfig
+++ b/openpower/configs/pseries_defconfig
@@ -15,7 +15,7 @@
 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="5.1.8"
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.1.15"
 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/pseries_skiroot_defconfig"