acpi-dma, doc: append managed function to the list

ACPI DMA provides managed function to register the slave DMA controller in the
internal container. This patch anounces that function in the corresponding
documentation file.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Rob Landley <rob@landley.net>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>