Automatically set DTSTimebase to PBUS frequency

[abejgonzalez]

See title.

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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[abejgonzalez]

Since the bootFreqHz parameter isn't used in downstream code, maybe just ignore it for now.