dmaengine: dw: move clock operations to platform.c

On BayTrail platform DMA is not functional in the PCI mode, whereby it always
failed and exit at the point when it tries to get a clock. It causes the PCI
mode probe to exit with the error message:
	dw_dmac_pci: probe of 0000:00:1e.0 failed with error -2

This patch moves clock operations to where it belongs to. Thus, the clock is
provided only in ACPI / non-PCI cases.

Reported-by: Chew, Chiau Ee <chiau.ee.chew@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>