refactor(st-clock): use refcnt instead of secure status

Rework the internal functions __stm32mp1_clk_enable/disable to check for
reference count instead of secure status for a clock.
Some functions now unused can be removed.

Change-Id: Ie4359110d7144229f85c961dcd5a019222c3fd25
Signed-off-by: Yann Gautier <yann.gautier@st.com>