fix(fconf): make struct fconf_populator static

In FCONF_REGISTER_POPULATOR macro, add static for the fconf_populator
struct. This avoids this kind of sparse warning:
plat/st/common/stm32mp_fconf_io.c:181:1: warning:
 symbol 'stm32mp_io__populator' was not declared. Should it be static?

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Icaa7da3079e82497e112585150f6348ef2ebf5e6