[ARM] 4780/1: S3C2412: Allow for seperate DMA channels for TX and RX

The current S3C24XX DMA code does not allow for an peripheral
that has one channel for RX and another for TX.

This patch adds a per-cpu dma operation to select the transmit
or receive channel, and adds support to the S3C2412 for the
seperate DMA channels for TX and RX.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>