feat(stm32mp1-fdts): use /omit-if-no-ref/ for pins nodes

feat(stm32mp1-fdts): use /omit-if-no-ref/ for pins nodes

With the /omit-if-no-ref/ keyword in DT, the non-referenced nodes
are just removed. This allows reducing the size of device tree blobs.
Setting it before pins node allows a size reduction of more than 2kB.
The corresponding nodes can also be removed from BL2 and BL32 DT
overlays.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I6b4a4d227d5592e1d253a1b35da2dafaac2ddcae