Bump skiboot to 5.3.4 for 5.3.x platforms: Garrison, Firenze and p9 mambo

Not cherry picked from master/master-next as we've already moved to a
base version of 5.3.x there, while op-build v1.10.y has some platforms
on 5.3.x, some on the old skiboot stable release.

Signed-off-by: Stewart Smith <stewart@linux.vnet.ibm.com>
diff --git a/openpower/configs/garrison_defconfig b/openpower/configs/garrison_defconfig
index 0d4dfce..8ebc28f 100644
--- a/openpower/configs/garrison_defconfig
+++ b/openpower/configs/garrison_defconfig
@@ -33,7 +33,7 @@
 
 # skiboot requirements
 BR2_SKIBOOT_CUSTOM_VERSION=y
-BR2_SKIBOOT_CUSTOM_VERSION_VALUE="skiboot-5.3.3"
+BR2_SKIBOOT_CUSTOM_VERSION_VALUE="skiboot-5.3.4"
 
 # petitboot requirements
 BR2_ENABLE_LOCALE_PURGE=y