char: virtio_fastrpc: use DMA_ATTR_SKIP_CPU_SYNC for uncached buffer

When buffer is uncached, use flag DMA_ATTR_SKIP_CPU_SYNC to
reduce cache sync time.

Change-Id: Icce9cd40fc6f3cd6aba69443c593b3947d8c6fdb
Signed-off-by: Ping Jiang <scottjiang@codeaurora.org>