fix(st-crypto): remove platdata functions

The functions stm32_pka_get_platdata() and stm32_saes_get_platdata()
are not used. They can be removed as we always use DT to retrieve
the device configuration.
This issue was triggered by sparse tool or when enabling warning
-Wmissing-prototypes.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I5cce9a0765144d694e8ddece37361ccbb85d1734