fix(st): explicitly check operators precedence

This corrects the MISRA violation C2012-12.1:
The precedence of operators within expressions should be made explicit.
This is done either by adding parentheses, or by creating dedicated
variables to ease readability.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I5e3f191ee38eca7ef634bd7542e615ab625271f6
diff --git a/plat/st/stm32mp1/stm32mp1_fconf_firewall.c b/plat/st/stm32mp1/stm32mp1_fconf_firewall.c
index f2568ab..7d99564 100644
--- a/plat/st/stm32mp1/stm32mp1_fconf_firewall.c
+++ b/plat/st/stm32mp1/stm32mp1_fconf_firewall.c
@@ -99,15 +99,16 @@
 
 	/* Locate the memory cells and read all values */
 	for (i = 0U; i < (unsigned int)(len / (sizeof(uint32_t) * STM32MP_REGION_PARAMS)); i++) {
+		uint32_t idx = i * STM32MP_REGION_PARAMS;
 		uint32_t base;
 		uint32_t size;
 		uint32_t sec_attr;
 		uint32_t nsaid;
 
-		base = fdt32_to_cpu(conf_list->id_attr[i * STM32MP_REGION_PARAMS]);
-		size = fdt32_to_cpu(conf_list->id_attr[i * STM32MP_REGION_PARAMS + 1]);
-		sec_attr = fdt32_to_cpu(conf_list->id_attr[i * STM32MP_REGION_PARAMS + 2]);
-		nsaid = fdt32_to_cpu(conf_list->id_attr[i * STM32MP_REGION_PARAMS + 3]);
+		base = fdt32_to_cpu(conf_list->id_attr[idx]);
+		size = fdt32_to_cpu(conf_list->id_attr[idx + 1]);
+		sec_attr = fdt32_to_cpu(conf_list->id_attr[idx + 2]);
+		nsaid = fdt32_to_cpu(conf_list->id_attr[idx + 3]);
 
 		VERBOSE("FCONF: stm32mp1-firewall cell found with value = 0x%x 0x%x 0x%x 0x%x\n",
 			base, size, sec_attr, nsaid);