dm: clk: add stub when CONFIG_CLK is deactivated

Add stub for functions clk_...() when CONFIG_CLK is deactivated.

This patch avoids compilation issues for driver using these API
without protection (#if CONFIG_IS_ENABLED(CLK))

For example, before this patch we have undefined reference to
`clk_disable_bulk') for code:
  clk_disable_bulk(&priv->clks);
  clk_release_bulk(&priv->clks);

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>