parisc: Improve cache flushing for PCXL in arch_sync_dma_for_cpu()

Add comment in arch_sync_dma_for_device() and handle the direction flag in
arch_sync_dma_for_cpu().

When receiving data from the device (DMA_FROM_DEVICE) unconditionally
purge the data cache in arch_sync_dma_for_cpu().

Signed-off-by: Helge Deller <deller@gmx.de>