stm32mp1: save boot interface in a static variable

In stm32_get_boot_interface(), there is no need to read the backup register
in which the boot interface and instance is saved. It can be saved in a
static variable. It will save time if it is used several times.

Change-Id: Ida5cbfeb9449d92bdb4f725eb6f6f1c4704282fe
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/188319


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>

plat/st/stm32mp1/stm32mp1_context.c

@@ -466,14 +466,17 @@ int stm32_save_boot_interface(uint32_t interface, uint32_t instance)
 
 int stm32_get_boot_interface(uint32_t *interface, uint32_t *instance)
 {
-	uint32_t itf;
-	uint32_t bkpr = tamp_bkpr(TAMP_BOOT_ITF_BACKUP_REG_ID);
+	static uint32_t itf;
 
-	clk_enable(RTCAPB);
+	if (itf == 0U) {
+		uint32_t bkpr = tamp_bkpr(TAMP_BOOT_ITF_BACKUP_REG_ID);
 
-	itf = (mmio_read_32(bkpr) & TAMP_BOOT_ITF_MASK) >> TAMP_BOOT_ITF_SHIFT;
+		clk_enable(RTCAPB);
 
-	clk_disable(RTCAPB);
+		itf = (mmio_read_32(bkpr) & TAMP_BOOT_ITF_MASK) >> TAMP_BOOT_ITF_SHIFT;
+
+		clk_disable(RTCAPB);
+	}
 
 	*interface = itf >> 4;
 	*instance = itf & 0xFU;