[POWERPC] 8xx: generic 8xx code arch/powerpc port

Including support for non-coherent cache, some mm-related things +
relevant field in Kconfig and Makefiles. Also included rheap.o compilation
if 8xx is defined.

Non-coherent mapping were refined and renamed according to Cristoph
Hellwig. Orphaned functions were cleaned up.

[Also removed arch/ppc/kernel/dma-mapping.c, because otherwise
compiling with ARCH=ppc for a non DMA-cache-coherent platform ends up
with two copies of __dma_alloc_coherent etc.
 -- paulus.]

Signed-off-by: Vitaly Bordug <vbordug@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>