Add MIPS boot support for the BCM4706/Northstar ChipCommon core.

This adds support for matching against a core lookup table when performing
early boot core lookup, and includes the BCM4706/Northstar-specific
ChipCommon core ID in the set of supported ChipCommon cores.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D10033