ARM64: dts: meson-gxl-libretech-cc: Add GPIO lines names

This patch describes the GPIO lines usage on the LibreTech CC board.

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

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
[khilman: minor whiespace fix]
Signed-off-by: Kevin Hilman <khilman@baylibre.com>