m68k: add support for DMA_BIDIRECTIONAL in dma support functions

The dma cache support functions do not currently support the direction flag
DMA_BIDIRECTIONAL. If a driver passes this direction to dma_map_single or
friends you will get console output like this:

  dma_sync_single_for_device: unsupported dir 0

For example when using the Intel e100 ethernet driver on a ColdFire platform
with PCI bus. You will get a stream of these messages coming out.

Modify the dma cache support code adding support for DMA_BIDIRECTIONAL. It is
actioned by doing a cache push operation.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>