feat(stm32mp1): add support for reading the metadata partition

Add support for reading the FWU metadata partition. The metadata
partition stores information on the current active bank along with
information on all the FWU updatable images on the platform. This
information is then used to identify the image to be booted.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: I66bc5ac718c21a49c504e698b5b1f5c4daed2d08