ARM: ux500: Rename the DT compatible entry for i2c devices on Snowball

It is believed that the i2c compatible match entry is too long
and cumbersome in its current form. This patch takes steps to
decrease its length by using ST-Microelectronics' abbreviation ST.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>