fwu: gpt: implement read_mdata and write_mdata callbacks

Moving towards using common code for meta-data management,
implement the read/write mdata hooks.

Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Sughosh Ganu <sughosh.ganu@linaro.org>