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