Add a mechanism to configure the source clock for UART

[SergioGasquez]

In espflash, we use UART for estimating the crystal frequency, in esp-hal, we use APB clock (hardcoded) as source for UART.

To generate the stubs, I had to update esp-hal, to use xtal_clock as source of UART, see changes in: https://github.com/esp-rs/esp-hal/compare/main...SergioGasquez:esp-hal:feat/clk-src. In order to avoid doing this every time we update the stubs (although it would only affect C2) , we want to add a way to configure the UART src clock.

[SergioGasquez]

Here is a list of the registers used to configure the clock source in UART for the different targets:

• ESP32 (UART_TICK_REF_ALWAYS_ON):
  • 0: REF_TICK
  • 1: APB clock
• ESP32-C2 (UART_SCLK_SEL):
  • 1: PLL_F40M_CLK
  • 2: RC_FAST_CLK
  • 3: XTAL_CLK
• ESP32-C3 (UART_SCLK_SEL):
  • 1: APB_CLK
  • 2: RC_FAST_CLK
  • 3: XTAL_CLK
• ESP32-C6
  • HP(PCR_UART0_SCLK_SEL/PCR_UART1_SCLK_SEL)
    • 0: Not select any clock
    • 1: Select PLL_F80M_CLK
    • 2: Select RC_FAST_CLK
    • 3: Select XTAL_CLK
  • LP:
• ESP32-H2 (PCR_UART0_SCLK_SEL/PCR_UART1_SCLK_SEL)
  • 0: No clock source
  • 1: PLL_F48M_CLK
  • 2: RC_FAST_CLK
  • 3 (default): XTAL_CLK
• ESP32-S2 (UART_TICK_REF_ALWAYS_ON)
  • 0: REF_TICK
  • 1: APB_CLK
• ESP32-S3 (UART_SCLK_SEL)
  • 1: APB_CLK
  • 2: RC_FAST_CLK
  • 3: XTAL_CLK