ARM: ux500: Enable LED heartbeat functionality on Snowball

This patch configures the user LED on Snowball to double flash
every second or so, whilst it's still alive. This can give key
indications as to what the board is doing in the case of no
console output.

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