drivers: dma: stm32: dmamux: abstact calls to dma drivers

Wrap calls to DMA drivers through fops to allow
different dmamuxes to point to different types of
dma with different function calls.

In preperation to add support for BDMA and DMAMUX2.

Co-authored-by: Jeroen van Dooren <jeroen.van.dooren@nobleo.nl>
Signed-off-by: Hein Wessels <heinwessels93@gmail.com>