docs(fwu): add firmware update documentation

Added firmware update documentation for:
1. PSA firmware update build flag
2. Porting guidelines to set the addresses of FWU metadata image
   and updated components in I/O policy

Change-Id: Iad3eb68b4be01a0b5850b69a067c60fcb464f54b
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
3 files changed