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_syscfg.c b/plat/st/stm32mp1/stm32mp1_syscfg.c
index ff79428..75dd709 100644
--- a/plat/st/stm32mp1/stm32mp1_syscfg.c
+++ b/plat/st/stm32mp1/stm32mp1_syscfg.c
@@ -235,7 +235,9 @@
 	}
 
 	if (apply_hslv) {
-		mmio_write_32(SYSCFG_BASE + SYSCFG_HSLVEN0R + index * sizeof(uint32_t), HSLV_KEY);
+		uint32_t reg_offset = index * sizeof(uint32_t);
+
+		mmio_write_32(SYSCFG_BASE + SYSCFG_HSLVEN0R + reg_offset, HSLV_KEY);
 	}
 }
 #endif