Comment 4 for bug 1941622

In commit 2113c0045c2a ("riscv: dts: add fu740 support") we have

   assigned-clock-rates = <1200000000>;

According to communication by SiFive this is the highest frequency that they can guarantee all boards to support.

u-boot-2021.01+dfsg uses

    assigned-clock-rates = <1001000000>;