[POWERPC] Fix build problem in ppc4xx_sgdma.c

ppc4xx_sgdma.c is #including asm/dma-mapping.h directly, which should
only ever be included via linux/dma-mapping.h.  asm/dma-mapping.h
relies on an enum defined in linux/dma-mapping.h before its own
include.  This fixes the problem.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>