[PATCH v3 0/2] i2c: add DMA support for freescale i2c driver

[Yuan Yao]

Changed in v3:
- fix a bug when request the dma faild.
- some minor fixes for coding style.
- other minor fixes.

Changed in v2:
- remove has_dma_support property
- unify i2c_imx_dma_rx and i2c_imx_dma_tx
- unify i2c_imx_dma_read and i2c_imx_pio_read
- unify i2c_imx_dma_write and i2c_imx_pio_write

Added in v1:
- Enable dma if it's support dma and transfer size bigger than the threshold.
- Add device tree bindings for i2c eDMA support.
- Add eDMA support for i2c driver.