clk: Move debug_node field under DEBUG_FS flag in struct clk_core

The debug_node field is only used when DEBUG_FS config is selected,
so declare it only if DEBUG_FS is selected.

Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>