clk: st: clkgen-pll: Demote unpopulated kernel-doc header

clk: st: clkgen-pll: Demote unpopulated kernel-doc header

And remove an incorrect entry.

Fixes the following W=1 kernel build warning(s):

 drivers/clk/st/clkgen-pll.c:142: warning: cannot understand function prototype: 'struct clkgen_pll '

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Stephen Gallimore <stephen.gallimore@st.com>
Cc: Pankaj Dev <pankaj.dev@st.com>
Cc: linux-clk@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210120093040.1719407-12-lee.jones@linaro.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>