Fix DefaultClockFrequencyKey rounding

[sant0s12]

This pull request fixes the frequency rounding for DefaultClockFrequencyKey. Before this change, setting the PBus frequency to a non integer value and using the WithPeripheryBusFrequencyAsDefault trait would make the default clock frequency and the PBus frequency differ when they should be the same.

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

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• Did you delete any extraneous prints/debugging code?
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