Oscillators and PLLs are not gated on stm32mp_clk_enable/disable()
calls. This change prevents functions to panic when called for such
always-on clocks. Gating these clocks is out of the scope of
this change.

Change-Id: Ie730553dea480b529de942446176db9119587832
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>