powerpc/powernv: Enable detection of legacy UARTs

Legacy UARTs can exist on PowerNV, memory-mapped ones on PCI
or IO based ones on the LPC bus.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/powernv/Kconfig

@@ -8,6 +8,7 @@ config PPC_POWERNV
 	select PPC_PCI_CHOICE if EMBEDDED
 	select EPAPR_BOOT
 	select PPC_INDIRECT_PIO
+	select PPC_UDBG_16550
 	default y
 
 config POWERNV_MSI