backlight: Add driver for iPAQ micro backlight

This adds a driver for the backlight controlled by the microcontroller
on the Compaq iPAQ series of handheld computers: h3100, h3600
and h3700.

Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>