[POWERPC] Support for the Ebony 440GP reference board in arch/powerpc

This adds platform support code for the Ebony (440GP) evaluation
board.  This includes both code in arch/powerpc/platforms/44x for
board initialization, and zImage wrapper code to correctly tweak the
flattened device tree based on information from the firmware.  The
zImage supports both IBM OpenBIOS (aka "treeboot") and old versions of
uboot which don't support a flattened device tree.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>