ARM: dts: sun6i-a31s-sinovoip-bpi-m2: add gpio-line-names

Add gpio-line-names as documented on gitbooks [1] and in the
schematics [2].

[1]: https://bananapi.gitbook.io/bpi-m2/en/bpi-m2_gpio_pin_define
[2]: https://drive.google.com/file/d/0B4PAo2nW2KfnRERWNnJGSGxJbmM/view



Signed-off-by: Michael Klein <michael@fossekall.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20201202195144.2105036-1-michael@fossekall.de