soc: stm32: Remove unused soc_register header file

soc_register.h is available in most of stm32 series.
It is sometime empty or contains includes to flash_registers.h,
which is included directly with direct reference in flash driver.
soc_register.h is also included in stm32 clock drivers, but this
driver do not use direct reference to any register anymore.

Clean up these files.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>