soc/rtc: update frequency switching APIs to match the master branch

esp32s2 code was based in IDF v3.1, and used outdated APIs.

Closes IDF-670

components/esp32s2/panic.c

@@ -476,7 +476,7 @@ static void esp_panic_dig_reset(void)
     // make sure all the panic handler output is sent from UART FIFO
     uart_tx_wait_idle(CONFIG_ESP_CONSOLE_UART_NUM);
     // switch to XTAL (otherwise we will keep running from the PLL)
-    rtc_clk_cpu_freq_set(RTC_CPU_FREQ_XTAL);
+    rtc_clk_cpu_freq_set_xtal();
     // reset the digital part
     esp_cpu_unstall(PRO_CPU_NUM);
     SET_PERI_REG_MASK(RTC_CNTL_OPTIONS0_REG, RTC_CNTL_SW_SYS_RST);