ARM: ux500: Do not attempt to register non-existent i2c devices on Snowball

This patch prevents i2c devices which are not present on the Snowball
low-cost development board from being registered. Devices such as;
tc3589x, bu1780 and lp5521 are present on other supported boards,
but are not located on Snowball.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>