stmhal: Move HAL Cube files to f4/ subdir, keeping only those we use.

This is in preparation for supporting other MCU series, such as
STM32F2xx.  Directory structure for the HAL is now hal/f4/{inc,src},
where "f4" will in the future be different for other series.

HAL source/header files that are not use are removed to reduce the size
of the code.