drivers: dmamux_stm32: compile time dmamux channels initialization

Move dmamux_stm32_channels struct from device data to device
configuration and initialize the dmamux channel - dma channel
assignment during build time,
as this association is hardwired in all known series.
The information is taken from dma_offset and dma_requests device tree
properties. The current implementation is valid for series with either
a single or two dma peripherals and a single dmamux peripheral.
Both dmas can independently enabled.
As the driver uses multi-instance DT_INST_DEFINE, also macros for a
second dmamux are are added that should allow easier extension to a
second dmamux instance.

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