clk_tree_rtc_slow_calibration() function

Performs a frequency calibration to RTC slow clock slowclk_cycles Number of slow clock cycles to count. If slowclk_cycles = 0, calibration will not be performed. Clock's theoretical value will be used. Returns the number of XTAL clock cycles within the given number of slow clock cycles It returns 0 if calibration failed, i.e. clock is not running