soc/stm32: add initial support for STM32F3X series

Supported SoCs:
- STM32F303XC
- STM32F334X8
- STM32F373XC

Change-Id: I74b9561272d54e270ae1a1f602fd389d4c86539a
Signed-off-by: Adam Podogrocki <adam.podogrocki@rndity.com>
14 files changed