dts: arm: st: stm32g0: 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. This avoids the
current situation where the same #include directive is duplicated among
many files, and where the file is sometimes included multiple times when
walking up the inclusion chain up until the root DTSI file.

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