ramips: enhance dma engine support

ramips: enhance dma engine support

* fix compiler error. device_control operation not support now.
* add old chips support 8 channels. new chips support 16 channels.
* add mt7621 hsdma driver. data sheet says it has two channels.
  but after test only one channel support.
* add memory to memory DMA support. i use dmatest kernel module to
  verify this function. on rt305x it will copy more data. on mt7621
  only two channels can works at the same time. these two chips
  maybe have hardware bugs. because on other chips don't have these bugs.
* use tasklet to handle remaining dma requests.

Signed-off-by: Michael Lee <igvtee@gmail.com>