Fix timer on all platforms

[JohnAZoidberg]

The countdown in the shell to run autostart.nsh says it's counting down from 5s but it's much faster.

• U540/U500 (works when setting the PCD to 10x the actual value, which is 1_000_000)
• RiscvVirt
• BeagleV (not tested but frequency is 6_250_000 according to datasheet)

[JohnAZoidberg]

Other platforms still use the SiFive timer, too.
They shouldn't.

[JohnAZoidberg]

On U540 it seems to be off by a factor of 10.
If I change PcdRiscVMachineTimerFrequencyInHerz from 1_000_000 to 10_000_000 the countdown to skip startup.sh seems to be roughly 5 seconds.
However the device tree of the U540 indicates that the frequency is 1_000_000.

Maybe there's something wrong in Silicon/RISC-V/ProcessorPkg/Library/RiscVTimerLib/RiscVTimerLib.c?