drivers: i2c: stm32: Use device tree as single source of compatibility

Some text in help section was documenting the compatibility of driver
variants with stm32 series.
This is duplicate of information versus device tree which should be
used as single source of information for hardware description.
Remove these lines, so we don't have to touch this driver each time
a series is added.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>