drivers, pch_dma: Fix warning when CONFIG_PM=n.

When CONFIG_PM=n, we get the following warning:

drivers/dma/pch_dma.c:741: warning: ‘pch_dma_suspend’ defined but not used
drivers/dma/pch_dma.c:755: warning: ‘pch_dma_resume’ defined but not used

To fix it, wrap pch_dma_{suspend,resume} and
pch_dma_{save,restore}_regs functions with CONFIG_PM.

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>