[mips/broadcom] Generic platform_reset() support.

This adds support for performing platform_reset() on all supported
devices, using early boot enumeration of chipc capabilities and
available cores.

• Added Broadcom-specific MIPS CP0 register definitions used by BCM4785-specific reset handling.
• Added a bcm_platform structure for tracking chipc/pmu/cfe platform data.
• Extended the BCMA EROM API to support early boot lookup of core info (including port/region mappings).
• Extended platform_reset() to support PMU, PMU+AOB, and non-PMU devices.

Reviewed by: mizhka
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D7539