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>