powerpc/dart: Use a cachable DART

Instead of punching a hole in the linear mapping, just use normal
cachable memory, and apply the flush sequence documented in the
CPC625 (aka U3) user manual.

This allows us to remove quite a bit of code related to the early
allocation of the DART and the hole in the linear mapping. We can
also get rid of the copy of the DART for suspend/resume as the
original memory can just be saved/restored now, as long as we
properly sync the caches.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[mpe: Integrate dart_init() fix to return ENODEV when DART disabled]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>