[mips/broadcom] Replace static frequency table with generic PMU clock handling.

[landonf]

Summary

Changes:

• Extended PWRCTL/PMU APIs to support querying clock frequency during very early boot, prior to bus attach.
• Implement generic PMU-based calculation of UART rclk values.
• Replaced use of static frequency tables (bcm_socinfo) with runtime-determined values.

Depends on D7492

Test Plan

For RT-N16, RT-N53, and WGT634U:

• Verified computed clock values against those specified in static table.
• Verified that I did not, in fact, break the serial console.

[landonf]

• Avoid using implicit global variables in BCM register macros. In theory, all the bcm_platform APIs should be usable with an arbitrary bcm_platform instance.
• PMU watchdog writes were always sent to ChipCommon, even if a PMU core was found. Since none of our currently supported devices use a distinct PMU core (and map the register block to ChipCommon), this worked fine.
• Due to a typo, the UART clock divisor register offset (instead of value) was improperly masked.