mailbox: stm32_ipcc: Convert to use APIs which support live DT

Use ofnode_ or dev_ APIs instead of fdt_ and fdtdec_ APIs so that the
driver can support live DT.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>