arm64: dts: imx8mq-librem5-r2: set nearlevel to 120

On Birch I can never reach 220 and hence the display would never
turn off. Tests suggest 120 to be a good threshold value for all Birch
devices.

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