H4 stops using gpio_expander_omap

This patch starts updating the H4 board support to use gpiolib.  It
switches from the gpio_expander_omap.c code to the mainline pcf8474
driver for its three I2C GPIO expanders.

It also adds two minor features:  LCD stays powered off when it's not
used; and the backlight control is exposed through the LED framework
(using the new "backlight" trigger).

Plus it fixes a bug that would have affected anyone trying to use the
camera sensor:  it wouldn't power up, since 0x80 != 0x08.  (Though
there's no mainline V4L2 driver for that sensor yet...)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>