boards/stm32f769i_disco: fix NOR flash max frequency

According to the datasheet, fRSCLK (Clock Frequency for READ instructions)
is 66MHz; it doesn't mention 72MHz anywhere.

Signed-off-by: Armin Brauns <[email protected]>

Author: arbrauns

boards/st/stm32f769i_disco/stm32f769i_disco.dts

@@ -175,7 +175,7 @@ arduino_serial: &usart6 {};
 	mx25l51245g: qspi-nor-flash@90000000 {
 		compatible = "st,stm32-qspi-nor";
 		reg = <0x90000000 DT_SIZE_M(64)>; /* 512 Mbits */
-		qspi-max-frequency = <72000000>;
+		qspi-max-frequency = <DT_FREQ_M(66)>;
 		status = "okay";
 
 		partitions {