drivers: clock_control: nrf: Fix waiting for LF clock

Algorithm for waiting for clock stabilization was failing in case when
it was waiting for clock availablity and clock was already available
before function was called. That is because nrfx_clock_is_running
was returning false because XTAL was already started but not yet
running.

Added a check for current LF source, if XTAL is picked that indicates
that RC is already running because of two stage startup procedure.

Added documentation of lfclk_spinwait with explanation of two stage
approach.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>