Marvell: Armada7k8k/OcteonTx: Fix RT debug prints

Resolution of the DebugLib for the DXE_RUNTIME_DRIVER library
class was limited to non-RELEASE builds. This caused crashes
during FWTS in case the RT attempted to use UART. Fix that
by allowing to use DxeRuntimeDebugLibSerialPort in all kind
of builds.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-By: Samer El-Haj-Mahmoud <Samer.El-Haj-Mahmoud@arm.com>
Reviewed-by: Sunny Wang <sunny.wang@arm.com>