serial: stm32: remove watchog reset in debug putc

serial: stm32: remove watchog reset in debug putc

For STM32MP, the watchdog is based on DM and the function watchod_reset
call the function uclass_get_device(UCLASS_WDT) to found the driver
associated IWDG2.

As this reset is not mandatory in debug putc (the  uart fifo will be
empty after some us), we can simplify the code by removing this call.

And this patch avoid issue when putc is called before initialization
of DM core, before the parsing of the device tree parsing and each
node bound to driver; that also avoid memory leak.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>