dma: define and document the source and dest adjust enum.
dma: define and document the source and dest adjust enum.

This is used in #6306 to make the driver more readable.

Signed-off-by: Michael Hope <c71d7810e3c04e4d6ad9df326b82536b86a1bfbc@google.com>

Vudentz/zephyr,kraj/zephyr,kraj/zephyr,mbolivar/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,ldts/zephyr,mbolivar/zephyr,finikorg/zephyr,Vudentz/zephyr,kraj/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,ldts/zephyr,kraj/zephyr,finikorg/zephyr,explora26/zephyr,galak/zephyr,zephyriot/zephyr,punitvara/zephyr,ldts/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,zephyriot/zephyr,punitvara/zephyr,nashif/zephyr,mbolivar/zephyr,mbolivar/zephyr,galak/zephyr,galak/zephyr,zephyriot/zephyr,explora26/zephyr,punitvara/zephyr,explora26/zephyr,explora26/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr