rtc_clk_cpu_set_to_default_config() function

Switch CPU clock source to XTAL, and let cpu frequency equal to main XTAL frequency. This function does not disable CPU's source PLL. If the PLL requires to be disabled to save power, please call `rtc_clk_cpu_freq_set_xtal` instead. It will always disable the corresponding PLL after switching the CPU clock source to XTAL (except for S2). Currently, this function is only called in `esp_restart_noos` and `esp_restart_noos_dig` to switch the CPU clock source back to XTAL (by default) before reset, and in `esp_sleep_start` to switch CPU clock source to XTAL before entering sleep for PMU supported chips.