asm-ppc64/dart.h is included in exactly one place -
arch/powerpc/sysdev/u3_iommu.c.  This patch, therefore, moves it into
arch/powerpc/sysdev.  While we're at it, update the #ifndef/#define
protecting the include, and the filename in the comments of
u3_iommu.c.

Built and booted on pSeries and G5, built for ppc32 powermac.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>