clk: exynos-audss: allow input clocks to be specified in device tree

This allows the input clocks to the Exynos AudioSS block to be
specified via device-tree bindings.  Default names will be used
when an input clock is not given.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Acked-by: Mike Turquette <mturquette@linaro.org>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Tomasz Figa <t.figa@samsung.com>