drivers: mcxc osc: use oscillator internal capacitance value from DT

Use default value 0 if board DT doesn't define it.e
It's totally fine if the board device tree doen't define
load_capacitance_picofarads for the crystal oscillator. In that case,
just fallback to 0.

Signed-off-by: jens rudberg <jens@teenage.engineering>