[POWERPC] Start arch/powerpc/boot code reorganization

This abstracts the operations used in the bootwrapper, and defines
the operations needed for the bootwrapper to run on an OF platform.

The operations have been divided up into platform ops (platform_ops),
firmware ops (fw_ops), device tree ops (dt_ops), and console ops
(console_ops).

The proper operations will be hooked up at runtime to provide the
functionality that you need.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>