OMAP3: RX-51: support sleep indicator LEDs

The sleep indicator LEDs can be enabled/disabled by toggling GPIO162.
Use the LED GPIO class to export this LED functionality to userspace.

To enable:

  # echo 1 > /sys/class/leds/sleep_ind/brightness

To disable:

  # echo 0 > /sys/class/leds/sleep_ind/brightness

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>