fbdev: imsttfb: remove the dependency on PPC_OF

The OF functionality has moved to a common place and be used by many
archs. So we don't need to depend on PPC_OF option any more. This is
a preparation for killing PPC_OF.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>