tests: drivers: clock_control: stm32_wl fix external clock dts

The nucleo_wl55jc according to the datasheet does have a
NT2016SF-32M-END5875A 32MHz TCXO as HSE, therefore needs
enable the "hse-tcxo;" property to work, this was not the case
for the clock_configuration/stm32_common_devices test cases.

Additionally, remove the comment about about ST-Link clock,
because the source is the tcxo and not the ST-Link.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>