ARM64: dts: meson-gxbb-odroidc2: Add GPIO lines names

This patch describes the GPIO lines usage on the Odroid-C2 board.

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

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>