arm64: dts: imx8mq-librem5: Add 166MHz to DDRC OPP table

This is the lowest frequency supported by older iMX8MQ SoC
revisions.
Signed-off-by: Sebastian Krzyszkowiak <sebastian.krzyszkowiak@puri.sm>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8mq-librem5.dtsi

@@ -329,6 +329,10 @@ opp-100000000 {
 			opp-hz = /bits/ 64 <100000000>;
 		};
 
+		opp-166000000 {
+			opp-hz = /bits/ 64 <166935483>;
+		};
+
 		opp-800000000 {
 			opp-hz = /bits/ 64 <800000000>;
 		};