PPC: E500: update u-boot to match shipped binary

Previous submodule commit contained some unused files with comments
that made redistribution requirements unclear, and the submodule commit
should really match the blob we're shipped anyway.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

diff --git a/roms/u-boot b/roms/u-boot
index 2072e7262965bb48d7fffb1e283101e6ed8b21a8..79c884d7e449a63fa8f07b7495f8f9873355c48f 160000 (submodule)
--- a/roms/u-boot
+++ b/roms/u-boot
@@ -1 +1 @@
-Subproject commit 2072e7262965bb48d7fffb1e283101e6ed8b21a8
+Subproject commit 79c884d7e449a63fa8f07b7495f8f9873355c48f