powerpc/dts: fix STMicroelectronics compatible strings

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

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Rob Herring <robh@kernel.org>