dmaengine: ioat: Move system_has_dca_enabled() to dma.h

dmaengine: ioat: Move system_has_dca_enabled() to dma.h

Move the function prototype for system_has_dca_enabled() from hw.h to
dma.h. This allows hw.h to be included from tools/, which will be used
in a subsysequent commit to implement a userspace driver for Intel CBDMA
devices in tools/testing/selftests/vfio.

No functional change intended.

Acked-by: Dave Jiang <dave.jiang@intel.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: David Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20250822212518.4156428-19-dmatlack@google.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>