dmaengine: stm32-dma3: use module_platform_driver

dmaengine: stm32-dma3: use module_platform_driver

Without module_platform_driver(), stm32-dma3 doesn't have a
module_exit procedure. Once stm32-dma3 module is inserted, it
can't be removed, marked busy.
Use module_platform_driver() instead of subsys_initcall() to register
(insmod) and unregister (rmmod) stm32-dma3 driver.

Reviewed-by: Eugen Hristev <eugen.hristev@linaro.org>
Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://patch.msgid.link/20251121-dma3_improv-v2-1-76a207b13ea6@foss.st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>