dmaengine: dw: platform: Mark 'hclk' clock optional

On some platforms the clock can be fixed rate, always running one and
there is no need to do anything with it.

In order to support those platforms, switch to use optional clock.

Fixes: f8d9ddbc ("dmaengine: dw: platform: Enable iDMA 32-bit on Intel Elkhart Lake")
Depends-on: 60b8f0dd

 ("clk: Add (devm_)clk_get_optional() functions")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20190924085116.83683-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>