ARM64: dts: meson-gxbb-nanopi-k2: Add GPIO lines names

This patch describes the GPIO lines usage on the Nanopi K2 board.

This is useful in the debugfs gpio file and using the cdev gpio API.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>