refactor(uart): add support to be able to test LP_UART port

Increase LP_UART_EMPTY_THRESH_DEFAULT value to 4. The original value
could cause the FIFO become empty before filling next data into the FIFO
when the buadrate is high. TX_DONE interrupt would raise before actual
transmission complete in such case.

components/soc/esp32/include/soc/clk_tree_defs.h

@@ -215,6 +215,11 @@ typedef enum {
 
 ///////////////////////////////////////////////////UART/////////////////////////////////////////////////////////////////
 
+/**
+ * @brief Array initializer for all supported clock sources of UART
+ */
+#define SOC_UART_CLKS {SOC_MOD_CLK_APB, SOC_MOD_CLK_REF_TICK}
+
 /**
  * @brief Type of UART clock source, reserved for the legacy UART driver
  */