ARM: ux500: Turn on the 'heartbeat' LED trigger

The heartbeat LED trigger provides an excellent debugging tool
when hacking on development boards. Here we enable it on all
u8500 based platforms. This will pulse the User LED on the
Snowball low-cost development board only.

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