feat(stm32mp1): add logic to pass the boot index to the Update Agent

With the FWU Multi Bank update feature, the platform can boot from one
of multiple banks(partitions). Pass the value of bank from which the
platform has booted as boot index to the Update Agent. The Update
Agent will match this boot index value against the active_index field
in the metadata, and update the metadata if there is a mismatch.

Fow now, the mechanism to pass the boot index is platform specific. On
the STM32MP1 platform, the boot index value is passed through a
memorey mapped TAMP register on the SoC.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: I0aa665ff9c1db95be8ae19ed8de6d866587d6850