arm64: dts: imx8mq-librem5: use multicolor leds description for RGB led

As Documentation/leds/well-known-leds.txt says, "Phones usually have
multi-color status LED." Fix that for the Librem 5 mobile phone board.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>