utils/hooks: Write offb device address to device tree

utils/hooks: Write offb device address to device tree

On OpenPOWER machines, kernels missing the AST video driver can use the
existing 'Open Firmware' framebuffer device code to inherit an existing
framebuffer from Petitboot. This requires us to translate the 64-bit
address of the framebuffer 'back' into a PCI address and write it into
an assigned-addresses property in the device tree.

Bootstrapped-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Sam Mendoza-Jonas <sam@mendozajonas.com>