Use bsec_find_otp_name_in_dt() to retrieve platform OTP information
from device tree, directly or through stm32_get_otp_index() and
stm32_get_otp_value() platform services.
String definitions replace hard-coded values, they are used to call this
new function.

Change-Id: I81213e4a9ad08fddadc2c97b064ae057a4c79561
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>