dma/ep93xx_dma.c: local symbols should be static

The symbol 'ep93xx_dma_prep_dma_memcpy' is only used in this driver
and should be marked static.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Mika Westerberg <mika.westerberg@iki.fi>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Acked-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>