stm32mp1: save boot information in backup registers

This will be used by BL33 to get boot device and instance.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Mathieu Belou <mathieu.belou@st.com>
Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
diff --git a/plat/st/stm32mp1/platform.mk b/plat/st/stm32mp1/platform.mk
index 46b7fbd..9c97f72 100644
--- a/plat/st/stm32mp1/platform.mk
+++ b/plat/st/stm32mp1/platform.mk
@@ -43,6 +43,7 @@
 				drivers/st/clk/stm32mp1_clkfunc.c			\
 				drivers/st/gpio/stm32_gpio.c				\
 				drivers/st/reset/stm32mp1_reset.c			\
+				plat/st/stm32mp1/stm32mp1_context.c			\
 				plat/st/stm32mp1/stm32mp1_dt.c				\
 				plat/st/stm32mp1/stm32mp1_helper.S