[siba] implement PMU behaviour for chips with a separate PMU module.

• PMU controls the clock setup
• Correctly set idle low power handling
• Use a hard-coded powerup delay for some of the newer hardware (including the BCM4322, which I've tested with.)

Tested:

• BCM4322, 2G + 5G STA mode

Obtained from: Linux b43 (PMU behaviour)