Review HSE clock initialization to match with latest CUBE firmware.
Also there is no need to set the full clock tree again after deep sleep exit.
With this change we get a stable deep sleep mode (when allowed by CORDIO stack).
- move hw_conf.h file to targets/TARGET_STM/TARGET_STM32WB directory as
this is used also out of BLE feature.
- create a dedicated hal_deepsleep function as the behavior in WB is a lot
different from other existing STM32 targets
- update clock tree configuration to directly clock the entire tree @ 32MHz
out of HSE. This is needed as we want to let the M0 core running without
any change on M0-side of clocks when M4 enters /exits deep sleep.