refactor(st-clock): directly use oscillator name

Instead of transmitting an 'enum stm32mp_osc_id', just send
directly the clock name with a 'const char *'

Change-Id: I866b05cbb1685a9b9f80e63dcd5ba7b1d35fc932
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>