ARM: ux500: Shorten Snowball's DT compatible gpio entry

It is believed that the gpio 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>