Platform/RaspberryPi/ConfigDxe: preliminary cleanup

Do some preliminary cosmetic cleanup on ConfigDxe before making
actual changes in a subsequent patch.

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