AT91: serial: Cherry-pick mainline commits for serial DMA support

At this time, the linux-3.10-at91 HEAD does not support DMA for
the USART or UART ports.  This commit pulls commits that were
cherry-picked from the mainline kernel that add serial DMA
support.  Verified to operate DMA (both TX and RX) on a
SAM9G25 board.

Signed-off-by: Bryan Evenson <bevenson@melinkcorp.com>