dmaengine: stm32: mark of_device_id table as maybe unused

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.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>