driver: dmamux: enable clocks only if node has clocks property defined

Not all STM32 Series can enable a dedicated clock for dmamux.
In stm32g0 series for example the clock is enabled automatically
as long as either DMA1 or DMA2 is enabled.
This commit changes dmamux driver to cope with socs that don't have
defined a clocks property. Therefore it moves the config(and data)
struct into the c file to be able to use DT_INST_NODE_HAS_PROP macro.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>