drivers: fwu: add the size parameter to the metadata access API's

drivers: fwu: add the size parameter to the metadata access API's

In version 2 of the metadata structure, the size of the structure
cannot be determined statically at build time. The structure is now
broken into the top level structure which contains a field indicating
the total size of the structure.

Add a size parameter to the metadata access API functions to indicate
the number of bytes to be accessed. This is then used to either read
the entire structure, or only the top level structure.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>