drivers: dma: stm32 dmamux device must be initialized after dma

Add specific init priority for the stm32 DMAMUX device higher
than the CONFIG_DMA_INIT_PRIORITY, to be sure that the DMAMUX
 initialization always comes after the stm32 DMA device init.
Its default value is set to 41 when the DMA_INIT_PRIORITY is
KERNEL_INIT_PRIORITY_DEFAULT (=40).

Signed-off-by: Francois Ramu <francois.ramu@st.com>