powerpc, dma: move bestcomm driver from arch/powerpc/sysdev to drivers/dma

The bestcomm dma hardware, and some of its users like the FEC ethernet
component, is used in different FreeScale parts, including non-powerpc
parts like the ColdFire MCF547x & MCF548x families.  Don't keep the
driver hidden in arch/powerpc where it is inaccessible for other arches.
.c files are moved to drivers/dma/bestcomm, while .h files are moved to
include/linux/fsl/bestcomm.  Makefiles, Kconfigs and #include directives
are updated for the new file locations.

Tested by recompiling for MPC5200 with all bestcomm users enabled.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Anatolij Gustschin <agust@denx.de>