dt/bindings: Add bindings for GPT based FWU Metadata storage device

Add bindings needed for accessing the FWU metadata partitions. These
include the compatible string which point to the access method and the
actual device which stores the FWU metadata.

The current patch adds basic bindings needed for accessing the
metadata structure on GPT partitioned block devices.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>