Documentation: dt: i2c: use correct STMicroelectronics vendor prefix

The documentation currently uses the non-standard vendor prefix stm
and st-micro for STMicroelectronics. The drivers do not specify the
vendor prefixes since the I2C Core strips them away from the DT
provided compatible string. Therefor, changing documentation and
existing device trees does not have any impact on device detection.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Rob Herring <robh@kernel.org>