ARM: OMAP: zoom: Use pwm stack for lcd and keyboard backlight

Use pwm_leds driver for the keyboard light and pwm-backlight for the lcd
backlight control (instead of implementing the PWM driver part in the board
file).

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>