ppc32: add Radstone PPC7D platform support

Radstone PPC7D are ppc7447A VME boards with Marvell Discovery-II,
dual GigE, dual PMC, 6 serial ports, keyboard/mouse, USB and optional
SCSI/VGA. This patch adds support for the PPC7D platform.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Mark A. Greer <mgreer@mvista.com>
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