arm64: dts: rockchip: rename LED label for NanoPi R4S

However "sys" is not a valid function, and it is always on.
Let's keep existing functions.

Fixes: db792e9a ("rockchip: rk3399: Add support for FriendlyARM NanoPi R4S")

Suggested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Tianling Shen <cnsztl@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Link: https://lore.kernel.org/r/20210426114652.29542-1-cnsztl@gmail.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>