i2c: sh_mobile: add DMA support

Make it possible to transfer i2c message buffers via DMA.
Start/Stop/Sending_Slave_Address is still handled using the old state
machine, it is sending the actual data that is done via DMA. This is
least intrusive and allows us to work with the message buffers directly
instead of preparing a custom buffer which involves copying the data
around.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
[wsa: fixed an uninitialized var problem]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>