ARM: bcm_defconfig: Enable PWM and Backlight

Enable PWM drivers and the PWM-based backlight driver.

Signed-off-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>