feat(st-clock): check HSE configuration in serial boot

In case of programmer mode, the bootrom manages to auto-detect
HSE clock configuration. In order to detect a bad device tree
setting in BL2, it will crash during programming if the configuration
is not aligned with the auto-detection.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Change-Id: I230697695745d6282d14b1ebfa6e4c4caa0cd8e2