stm32: dmamux: change initialization order to `PRE_KERNEL_1`

When running the tests of the async api of uart
on nucleo-l4r5zi we get MPU fault.
The reason is ordering initialization. The uart and dma
drivers are initialized PRE_KERNEL_1 while dmamux is
initialized POST_KERNEL.
Thus, the function device_get_binding fails since the
dmamux device is not ready to be used.

Fixes: #32715.

Signed-off-by: Shlomi Vaknin <shlomi.39sd@gmail.com>