Platforms/RaspberryPi: SMBIOS Type 0 fixes

Various fixes and enhancements for SMBIOS Type 0:
 - Use PCDs to report the BIOS Segment and Size
 - Report Extended BiosSize
 - Set BiosIsUpgradable (it is!)
 - Clear FunctionKeyNetworkBootIsSupported

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Pete Batard <pete@akeo.ie>
Cc: Andrei Warkentin <awarkentin@vmware.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Signed-off-by: Samer El-Haj-Mahmoud <samer.el-haj-mahmoud@arm.com>
Reviewed-by: Andrei Warkentin <awarkentin@vmware.com>