stm32mp1: Add a node for the FWU metadata device

The FWU metadata structure is accessed through the driver model
interface. On the stm32mp157c dk2 and ev1 boards, the FWU metadata is
stored on the uSD card. Add the fwu-mdata node on the u-boot specifc
dtsi file for accessing the metadata structure.

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>