feat(st-clock): manage disabled oscillator

Support "disabled" status for oscillator in device tree.

At boot time, the clock tree initialization performs the following
tasks:
- enabling of the oscillators present in the device tree and not
  disabled,
- disabling of the HSI oscillator if the node is absent or disabled
  (always activated by bootROM).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: I176276022334f3d97ba0250b54062f0ae970e239