[POWERPC] 86xx: fix guts_set_dmacr() and add guts_set_pmuxcr_dma() to immap_86xx.h

Updated guts_set_dmacr() to enumerate the DMA controllers at 0, instead of 1,
so that it now matches other related functions.  Added function
guts_set_pmuxcr_dma() to set the external DMA control bits in the PMUXCR
register of the global utilities structure.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>