Bump skiboot to 5.3.7 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/firenze_defconfig b/openpower/configs/firenze_defconfig
index 08f3c7c..61f4d09 100644
--- a/openpower/configs/firenze_defconfig
+++ b/openpower/configs/firenze_defconfig
@@ -26,7 +26,7 @@
 BR2_PACKAGE_SKIBOOT=y
 BR2_PACKAGE_SKIBOOT_EMBED_PAYLOAD=n
 BR2_SKIBOOT_CUSTOM_VERSION=y
-BR2_SKIBOOT_CUSTOM_VERSION_VALUE="skiboot-5.3.6"
+BR2_SKIBOOT_CUSTOM_VERSION_VALUE="skiboot-5.3.7"
 
 # petitboot requirements
 BR2_ENABLE_LOCALE_PURGE=y