ARM: nomadik: add led and key for S8815

This adds device tree hunks for the LED and userbutton on the
USB S8815 board, and set up a heartbeat trigger on the LED and
an escape key on the user button. Alter the defconfig to enable
these standard DT-enabled GPIO drivers.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>