i2c: aspeed: add DMA mode transfer support

This commit adds DMA mode transfer support.

Only AST2500 and later versions supports DMA mode.

AST2500 has these restrictions:
  - If one of these controllers is enabled
      * UHCI host controller
      * MCTP controller
    I2C has to use buffer mode or byte mode instead
    since these controllers run only in DMA mode and
    I2C is sharing the same DMA H/W with them.
  - If one of these controllers uses DMA mode, I2C
    can't use DMA mode
      * SD/eMMC
      * Port80 snoop

Signed-off-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>