i2c-stu300: Make driver depend on MACH_U300

This makes the stu300 driver for the ST Micro ST DDC I2C bus
driver depend on MACH_U300, new platforms reusing this I2C
driver will need to add in a similar dependency.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
[ben-linux@fluff.org: re-aranged subject line]
Signed-off-by: Ben Dooks <ben-linux@fluff.org>