Given that sun4u uses sparc64/sparc64/ofw_machdep.c, use the sparc64

Given that sun4u uses sparc64/sparc64/ofw_machdep.c, use the sparc64
<machine/ofw_machdep.h> here also. This isn't exactly a clean approach
though as ofw_machdep.h also includes some prototypes for functions
that aren't actually shared between both architectures but just happen
to have the same interface. Given that the sun4v support actually is
dead and this is just to allow things to continue to compile it should
be okay though.

This is a direct commit to stable/8.

git-svn-id: svn://svn.freebsd.org/base/stable/8@231239 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f