buildroot: Fix repo url, pull 5.9 kernel support

The buildroot sub-module was still mistakenly pointing towards my
personal repo, so fix that to point towards the open-power repo and also
pull the newest commit which adds kernel 5.9 support.

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
diff --git a/.gitmodules b/.gitmodules
index 5cda80d..0a83da8 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,4 +1,4 @@
 [submodule "buildroot"]
 	path = buildroot
 	branch = master-p10
-	url = git@github.ibm.com:klausk/buildroot.git
+	url = git@github.ibm.com:open-power/buildroot.git