[POWERPC] Add udbg support for RTAS console

Add udbg hooks for the RTAS console, based on the RTAS put-term-char
and get-term-char calls. Along with my previous patches, this should
enable debugging as soon as early_init_dt_scan_rtas() is called.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index c5e01c4..deea4c8e1 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -420,6 +420,10 @@
 	select MMIO_NVRAM
 	select PPC_UDBG_16550
 
+config UDBG_RTAS_CONSOLE
+	bool
+	default n
+
 config XICS
 	depends on PPC_PSERIES
 	bool