ARM: at91: at91sam9x5: use pwm device for lcd backlight

This patch is an example to use a pwm device for lcd backlight. Of
course it not a clean solution because it removes one red bit from the
lcd data. The good solution will be to switch the lcd to RGB 565, it
involves changing resistors on the display module.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>