ARM: dts: bcm2837-rpi-3-b: add GPIO expander

Add a description of the RPi3 GPIO expander that the VC4 firmware controls.

Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>