Once the ST frontend demux HW IP has been enabled, the clock can't
be disabled otherwise the system will hang and the board will
be unserviceable.

To allow balanced clock enable/disable calls in the driver we use
the critical clock infrastructure to take an extra reference on the
clock so the clock will never actually be disabled.
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Patrice Chotard <patrice.chotard@st.com>