ARM: ux500: Enable the user LED on Snowball via Device Tree

This patch enables and illuminates the user_led on the
Snowball low-cost development board using DT. It also
removes initialisation carried out from platform code.

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