ARM: dts: omap3-overo: Add support for pwm-leds

Convert the on-board LED connected to the TWL4030 (LEDB) to use
pwm-leds.

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Benoit Cousson <benoit.cousson@linaro.org>