ext/hal: stm32f2xx: Use of (__packed uint32_t *) produces warnings

Using Zephyr SDK 0.10.0-rc2, GNUCC 8.2.0 is used and
(__packed uint32_t *) are now generating warnings..
Replace with CMSIS macros __UNALIGNED_UINT32_READ and
__UNALIGNED_UINT32_WRITE

ST internal reference: 61324

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