ARM: ux500: Kconfig: Compile in leds-gpio support for Snowball

Code exists in the mop500 board file (used for HREF and Snowball)
to initialise Snowball's user LED via the leds-gpio driver. However,
the driver isn't currently built when using the current configuration.
This patch aims to change that behavior.

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