soc: efr32fg1p: correct clock initialization sequence

This patch changes clock initialization sequence to initialize external
cristal oscillators only if it was not done before. Initialization of
external cristal oscillators may be performed by the bootloader.

Tested on BRD4250B evaluation board.

Fixes #9471

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>