fix(uart): uses REF_TICK for ESP32/S2

Author: SuGlider

cores/esp32/esp32-hal-uart.c

@@ -507,9 +507,11 @@ uart_t *uartBegin(
   // therefore, uart clock source will set to XTAL for all SoC that support it. This fix solves the C6|H2 issue.
 #if SOC_UART_SUPPORT_XTAL_CLK
   uart_config.source_clk = UART_SCLK_XTAL; // valid for C2, S3, C3, C6, H2 and P4
+#elif SOC_UART_SUPPORT_REF_TICK
+  uart_config.source_clk = UART_SCLK_REF_TICK; // valid for ESP32, S2
 #else
   // Default CLK Source: CLK_APB for ESP32|S2|S3|C3 -- CLK_PLL_F40M for C2 -- CLK_PLL_F48M for H2 -- CLK_PLL_F80M for C6
-  uart_config.source_clk = UART_SCLK_DEFAULT; // valid for ESP32 and S2
+  uart_config.source_clk = UART_SCLK_DEFAULT; // baudrate may change with the CPU Frequency!
 #endif
 
   UART_MUTEX_LOCK();