[POWERPC] Make more OF-related bootwrapper functions available to non-OF platforms

Commit 2e601613 split up
arch/powerpc/boot/of.c so that some OF functions can be used on
platforms that don't want to use the overall OF platform boot code.
This is useful on things like PReP which can have an OF implementation
which is useful for debugging output, but inadequate for booting.

However, that commit didn't export quite enough things to make a
usable OF console on a non-OF system.  In particular, the device tree
manipulation performed to initialize the OF console code must
explicitly use the OF device tree, rather than the flattened device
tree, even if the system is otherwise booting using a flattened device
tree.  This makes it so.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>