Platforms/RaspberryPi: Fix DBG2 UART namespace reference

The UART namespace reference in DBG2 is incorrect. Fix to point to the
correct name.

This fixes the certification failure reported by FWTS tests at:
https://github.com/pftf/RPi4/issues/69



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: Pete Batard <pete@akeo.ie>