dmaengine: qcom: bam_dma: make bam clk optional

When BAM is remotely controlled it does not sound correct to control
its clk on Linux side. Make it optional, so that its not mandatory
for remote controlled BAM instances.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>