refactor(st-clock): improve DT parsing for PLL nodes

Add a function to get PLL settings from DT:
"cfg" property is mandatory, an error is generated if not found.
"frac" is optional, default value is returned if not found.
"csg" is optional too, a boolean value indicates if it has been
found, and its value is updated.

Store each PLL node validity information, this avoids parsing DT
several times.

Change-Id: I039466fbe1e67d160f7112814e7bb63b661804d0
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
1 file changed