Replace mips/sentry5 with mips/broadcom

Replace mips/sentry5 with mips/broadcom

The delta between SENTRY5 and BCM was already small due to BCM being
derived from SENTRY5; re-integrating the two avoids the maintenance
overhead of keeping them both in sync with bhnd(4) changes.

- Re-integrate minor SENTRY5 deltas in bcm_machdep.c
- Modify uart_cpu_chipc to allow specifying UART debug/console flags via
  kenv and device hints.
- Switch SENTRY5 to std.broadcom
- Enabled CFI flash support for SENTRY5

Reviewed by:	Michael Zhilin <mizkha@gmail.com> (Broadcom MIPS support)
Approved by:	re (gjb), adrian (mentor)
Differential Revision:	https://reviews.freebsd.org/D6897