dtoverlays: Add overlay for HD44780 via I2C PCF8574 backpack

Many HD44780 LCD displays are connected via very common I2C
GPIO expander.
We have an overlay for connecting the displays directly to GPIOs,
but not one for it connected via a backpack. Add such an overlay.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>