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>
diff --git a/plat/st/stm32mp1/plat_image_load.c b/plat/st/stm32mp1/plat_image_load.c
index 3c6d677..d5b328e 100644
--- a/plat/st/stm32mp1/plat_image_load.c
+++ b/plat/st/stm32mp1/plat_image_load.c
@@ -5,6 +5,7 @@
  */
 
 #include <desc_image_load.h>
+#include <platform.h>
 
 /*******************************************************************************
  * This function flushes the data structures so that they are visible