Platform/RaspberryPi/RPi3: Fix TFTP dynamic command initialization

Set the PcdShellLibAutoInitialize for the TFTP dynamic Shell command.
This fixes an ASSERT observed in https://github.com/pftf/RPi3/issues/11



Signed-off-by: Samer El-Haj-Mahmoud <samer@elhajmahmoud.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@arm.com>