dmaengine: stm32-mdma: check the channel availability (secure or not)

STM32_MDMA_CCR bit[8] is used to enable Secure Mode (SM). If this bit is
set, it means that all the channel registers are write-protected. So the
channel is not available for Linux use.

Add stm32_mdma_filter_fn() callback filter and give it to
__dma_request_chan (instead of dma_get_any_slave_channel()), to exclude the
channel if it is marked Secure.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://lore.kernel.org/r/20220330103645.99969-1-amelie.delaunay@foss.st.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>