rtc_clk_slow_freq_get_hz() function

Get the approximate frequency of RTC_SLOW_CLK, in Hz - if SOC_RTC_SLOW_CLK_SRC_RC_SLOW is selected, returns ~150000 - if SOC_RTC_SLOW_CLK_SRC_XTAL32K is selected, returns 32768 - if SOC_RTC_SLOW_CLK_SRC_RC_FAST_D256 is selected, returns ~33000 rtc_clk_cal function can be used to get more precise value by comparing RTC_SLOW_CLK frequency to the frequency of main XTAL.