doc: fwu: make changes to reflect support for FWU metadata v2

The FWU Update Agent in U-Boot supports both versions of the FWU
metadata. Make changes in the documentation to reflect this.

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