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:

• [x] Bug fix
• [ ] New feature
• [ ] Other enhancement

Impact:

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

Contributor Checklist:

• [x] Did you set main as the base branch?
• [x] Is this PR's title suitable for inclusion in the changelog and have you added a changelog:<topic> label?
• [x] Did you state the type-of-change/impact?
• [x] Did you delete any extraneous prints/debugging code?
• [ ] Did you mark the PR with a changelog: label?
• [ ] (If applicable) Did you add documentation for the feature?
• [ ] (If applicable) Did you add a test demonstrating the PR?

• [ ] (If applicable) Did you mark the PR as Please Backport?