stm32mp1: correct some static analysis tools issues

These issues wer found by sparse:

drivers/st/clk/stm32mp1_clk.c:1524:19:
 warning: incorrect type in assignment (different base types)
    expected restricted fdt32_t const [usertype] *pkcs_cell
    got unsigned int const [usertype] *

plat/st/stm32mp1/plat_image_load.c:13:6:
 warning: symbol 'plat_flush_next_bl_params' was not declared.
 Should it be static?
plat/st/stm32mp1/plat_image_load.c:21:16:
 warning: symbol 'plat_get_bl_image_load_info' was not declared.
 Should it be static?
plat/st/stm32mp1/plat_image_load.c:29:13:
 warning: symbol 'plat_get_next_bl_params' was not declared.
 Should it be static?

plat/st/stm32mp1/bl2_io_storage.c:40:10:
 warning: symbol 'block_buffer' was not declared. Should it be static?

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