ARM: dts: sun8i-h2-plus-bananapi-m2-zero: add gpio-line-names

Add gpio-line-names as documented in the Banana Pi wiki [1] and in the
schematics [2].

[1]: http://wiki.banana-pi.org/Banana_Pi_BPI-M2_ZERO#GPIO_PIN_define
[2]: https://drive.google.com/file/d/0B4PAo2nW2KfnMW5sVkxWSW9qa28/view



Signed-off-by: Michael Klein <michael@fossekall.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20201123114535.1605939-1-michael@fossekall.de