dts: arm: st: stm32c0: include <mem.h> from series' root DTSI

Include <mem.h> from the series' root DTSI file and remove inclusion of
the file from other levels since it becomes redundant.

There is no consistency about which level of the DTSI inclusion hierarchy
the macros from this header start being used, so including it from the root
file ensures they are always visible and available without headache. Today,
there are places where the header is included twice or more at different
DTSI inclusion hierarchy levels because of this ambiguity - this commit
solves the issue once and for all.

Signed-off-by: Mathieu Choplain <mathieu.choplain-ext@st.com>
11 files changed