Switch mips/sentry5 to bhnd(4), and unify with mips/broadcom

Now that bhnd(4) provides feature parity with the previous siba/mips
implementation, we can switch sentry5 over and begin lifting common
support code out for use across bhnd(4) embedded targets.

Changes:

• Fixed enumeration of siba(4) per-core address maps, required for discovery of memory mapped chipc flash region on siba(4) devices.
• Simplified bhnd kernel configuration (dropped 'bhndbus' option).
• Replaced files.broadcom's direct file references with their corresponding standard kernel options.
• Lifted out common bcma/siba nexus support, inheriting from the new generic bhnd_nexus driver.
• Dropped now-unused sentry5 siba code.
• Re-integrated BCM into the universe build now that it actually compiles.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D6712