Support switchable C2C links through the OBUS

[jerryz123]

The OBUS is a special bus that selects between multiple possible interfaces for a C2C link.
Outwards connections from the OBUS can be different interfaces. A select pin into the chip selects which outwards connection/interface is used after reset is deasserted.

A demo config shows how 2 different interfaces (variants of serial-TL) can be used in the same C2C link via this mechanism.

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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