fix(st): make metadata_block_spec static

This issue was triggered by sparse tool:
plat/st/common/stm32mp_fconf_io.c:31:17: warning:
 symbol 'metadata_block_spec' was not declared. Should it be static?

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I283d15792ed0e7ac5181e18aaf54010a0e61b370