Some targets don't support UART_SCLK_APB (e.g. esp32c2)
This change is applicable only for IDFv5.0 and later, as older version
didn't define UART_SCLK_DEFAULT.
Closes https://github.com/espressif/esp-protocols/issues/241
This is for example needed when using an external crystal to select
UART_SCLK_XTAL, which fixes uart comm issue when using PM with light
sleep enabled.