dmaengine: tegra-apb: mark PM functions as __maybe_unused

When CONFIG_PM is disabled, gcc warning this:

drivers/dma/tegra20-apb-dma.c:1587:12: warning: 'tegra_dma_runtime_resume' defined but not used [-Wunused-function]
drivers/dma/tegra20-apb-dma.c:1578:12: warning: 'tegra_dma_runtime_suspend' defined but not used [-Wunused-function]

Make it as __maybe_unused to fix the warnings,
also remove unneeded function declarations.

Fixes: ec8a1586 ("dma: tegra: add dmaengine based dma driver")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20200320071337.59756-1-yuehaibing@huawei.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/tegra20-apb-dma.c

@@ -263,8 +263,6 @@ static inline struct device *tdc2dev(struct tegra_dma_channel *tdc)
 }
 
 static dma_cookie_t tegra_dma_tx_submit(struct dma_async_tx_descriptor *tx);
-static int tegra_dma_runtime_suspend(struct device *dev);
-static int tegra_dma_runtime_resume(struct device *dev);
 
 /* Get DMA desc from free list, if not there then allocate it.  */
 static struct tegra_dma_desc *tegra_dma_desc_get(struct tegra_dma_channel *tdc)
@@ -1580,7 +1578,7 @@ static int tegra_dma_remove(struct platform_device *pdev)
 	return 0;
 }
 
-static int tegra_dma_runtime_suspend(struct device *dev)
+static int __maybe_unused tegra_dma_runtime_suspend(struct device *dev)
 {
 	struct tegra_dma *tdma = dev_get_drvdata(dev);
 
@@ -1589,7 +1587,7 @@ static int tegra_dma_runtime_suspend(struct device *dev)
 	return 0;
 }
 
-static int tegra_dma_runtime_resume(struct device *dev)
+static int __maybe_unused tegra_dma_runtime_resume(struct device *dev)
 {
 	struct tegra_dma *tdma = dev_get_drvdata(dev);