arm: dts: sun7i-a20-bananapi: name the GPIO lines

This names the GPIO lines on the Banana Pi board in accordance with
the A20_Banana_Pi v1.4 Specification.

This will make these line names reflect through to user space
so that they can easily be identified and used with the new
character device ABI.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>