FWU: STM32MP1: Add support to read boot index from backup register

The FWU Multi Bank Update feature allows the platform to boot the
firmware images from one of the partitions(banks). The first stage
bootloader(fsbl) passes the value of the boot index, i.e. the bank
from which the firmware images were booted from to U-Boot. On the
STM32MP157C-DK2 board, this value is passed through one of the SoC's
backup register. Add a function to read the boot index value from the
backup register.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
diff --git a/include/fwu.h b/include/fwu.h
index 5391af9..73628c5 100644
--- a/include/fwu.h
+++ b/include/fwu.h
@@ -341,4 +341,16 @@
  *
  */
 int fwu_plat_get_update_index(uint *update_idx);
+
+/**
+ * fwu_plat_get_bootidx() - Get the value of the boot index
+ * @boot_idx: Boot index value
+ *
+ * Get the value of the bank(partition) from which the platform
+ * has booted. This value is passed to U-Boot from the earlier
+ * stage bootloader which loads and boots all the relevant
+ * firmware images
+ *
+ */
+void fwu_plat_get_bootidx(uint *boot_idx);
 #endif /* _FWU_H_ */