[POWERPC] Only allow building of BootX text support on PPC_MULTIPLATFORM

BootX text code is only supported on systems with real OF at this point
which is PPC_OF && PPC_MULTIPLATFORM.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

arch/powerpc/Kconfig.debug

@@ -134,7 +134,7 @@ config BDI_SWITCH
 
 config BOOTX_TEXT
 	bool "Support for early boot text console (BootX or OpenFirmware only)"
-	depends PPC_OF
+	depends PPC_OF && PPC_MULTIPLATFORM
 	help
 	  Say Y here to see progress messages from the boot firmware in text
 	  mode. Requires either BootX or Open Firmware.