feat(stm32mp1): add support for building the FWU feature

Add support for enabling the FWU multi bank boot feature on the
platform.

Currently, this feature is supported on the STM32MP157C-DK2 board,
which boots off a uSD card. Also, support has been enabled when
booting from a FIP image.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: Ia69e858461e2daf599d41d66d7ff2ccae0c341c2