powerpc: Add support for OpenBlockS 600

powerpc: Add support for OpenBlockS 600

So I've had one of these for a while and it looks like the vendor never
bothered submitting the support upstream.

This adds it using ppc40x_simple and provides a device-tree.

There are some changes to the boot wrapper because the way u-boot works
on this thing, it seems to expect a multipart image with the kernel,
initrd and dtb in it.

The USB support is missing as it needs the yet unmerged driver for
the DWC OTG part and the GPIOs may need further definition in the dts.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>