Assign default roles to two of Beagle's programmable LEDs:

 - usr0 has a heartbeat to show basic activity
 - usr1 shows MMC activity

A third led is controlled by the TWL4030 LED_B signal, but the
TWL driver doesn't yet support leds...
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>