PPC: E500: Update u-boot to commit 79c884d7e4

The current U-Boot binary in QEMU has a bug where it fails to support
dynamic CCSR addressing. Without this support, u-boot can not boot the
ppce500 machine anymore. This has been fixed upstream in u-boot commit
e834975b.

Update the u-boot blob we carry in QEMU to the latest u-boot upstream,
so that we can successfully run u-boot with the ppce500 machine again.

CC: qemu-stable@nongnu.org
Signed-off-by: Alexander Graf <agraf@suse.de>
Tested-by: Thomas Huth <thuth@redhat.com>