soc/rtc: fix spurious warnings about XTAL frequency on startup

soc/rtc: fix spurious warnings about XTAL frequency on startup

1. Make sure that 8MD256 clock used to estimate XTAL frequency is enabled
   before trying to use rtc_clk_cal_ratio.
   This fixes "Bogus XTAL frequency: 0 MHz" warnings after software reset.

2. Don't call rtc_clk_xtal_freq_estimate if XTAL frequency is already
   known. This reduces startup time after deep sleep or software reset.

3. Compare known XTAL frequency and estimated one before printing a
   warning. This fixes "Possibly invalid CONFIG_ESP32_XTAL_FREQ setting
   (40MHz). Detected 40 MHz." warnings.