Reduce panda power usage

[elkoled]

Panda Power Optimization
Reduces OBD-C port power by 77% (225 mW -> 52 mW) in low power state.

The panda STM32H7 MCU on comma four runs at 240 MHz on VOS1 voltage scaling. In its stock configuration with all peripheral clocks enabled it draws 225 mW in low power state.
This can be reduced to 52 mW in 3 main steps:

• Disable clocks and power to peripherals we don't need in standby
• Drop voltage scaling from VOS1 (High Performance) to VOS3 (Low Power)
• Enter deep sleep

Each optimization was measured individually on comma four hardware.

SIL2 considerations:
Stop mode is only entered from SAFETY_SILENT when no safety function is active, fulfilling rule CoU_3 of UM2331.

[adeebshihadeh]

Can we also store a reset count or some other debug info in memory?

[elkoled]

There is a 32bit RTC backup register that could be used for saving a reset counter, it resets on VBAT power loss.

[elkoled]

NVIC_SystemReset wrapper and save reset counter is a good separate PR.