Support using HarnessBinders without IOBinders

[jerryz123]

This makes it easier to build systems with very-custom ChipTops that don't use IOBinders for whatever reason. Those ChipTops just need to instantiate a list of Port objects that the HarnessBinders attach to.

This also enables us to breakout ChipTop I/Os into individual pins, which can help in scenarios where we do not control top-level bump names in a VLSI flow.

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]

Neat. LGTM

[abejgonzalez]

LGTM