twl4030-gpio supports LED signals

Expose the two TWL4030 LED signals as output-only GPIOs.  Boards
need to explicitly ask that this be done, to help avoid conflicts
on boards using these same pins to hook up to a vibrator motor.

Note that these are high drive open drain signals; LEDA is rated
for up to 160 mA (!), LEDB up to 60 mA.  Boards using one of these
signals to drive a bank of LCD backlight LEDs would probably want
to access the dedicated PWMs for brightness control, too; easy to
add such support later.

Example:  Beagle has one real LED here (PWM not necessary), and
one GPIO controlling VBUS output over EHCI (PWM not wanted).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>