ARM64: dts: meson-gxl-khadas-vim: Add GPIO lines names

This patch describes the GPIO lines usage on the Khadas VIM 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 whitespace fix]
Signed-off-by: Kevin Hilman <khilman@baylibre.com>