[POWERPC] bootwrapper: Add ppcboot.h

This file describes the bd_t struct, which is used by old versions of
U-boot to pass information to the kernel.  Platform code that needs to
interoperate with such firmware can use this; it should not be used for
anything new.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>