muc/stm32: Add workaround for STM32F4x5/7 WFI error
muc/stm32: Add workaround for STM32F4x5/7 WFI error

MCUs from STM32F405, STM32F407, STM32F415, STM32F417 family
have documented error behaviour when exiting from sleep mode
when WFI instruction placed at address ending with xx4 was executed.

See en.DM00037591.pdf section 2.1.3 for details and conditions.

It is more cumbersome to make sure WFI is at safe location so
simplest workaround proposed by ST is implemented:
- Add 3 NOP instructions after WFI.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core