dmaengine: stm32: mark of_device_id table as maybe unused

The driver uses a second of_device_id table in the probe()
function by passing it to of_match_node().  This code will be a no-op
for compile testing (!CONFIG_OF on x86_64):

    drivers/dma/stm32-dmamux.c:171:34: warning:
        ‘stm32_stm32dma_master_match’ defined but not used [-Wunused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201120162303.482126-5-krzk@kernel.orgSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/stm32-dmamux.c

@@ -168,7 +168,7 @@ static void *stm32_dmamux_route_allocate(struct of_phandle_args *dma_spec,
 	return ERR_PTR(ret);
 }
 
-static const struct of_device_id stm32_stm32dma_master_match[] = {
+static const struct of_device_id stm32_stm32dma_master_match[] __maybe_unused = {
 	{ .compatible = "st,stm32-dma", },
 	{},
 };