powerpc/xmon: Select CONSOLE_POLL for the 8xx

Powerpc 8xx requires CONSOLE_POLL to get udbg_putc() and
udbg_getc() in CPM uart driver.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3d10a274516e9be8c4b0dc679a2840cdc1588872.1608716197.git.christophe.leroy@csgroup.eu

arch/powerpc/Kconfig.debug

@@ -88,6 +88,7 @@ config PPC_IRQ_SOFT_MASK_DEBUG
 config XMON
 	bool "Include xmon kernel debugger"
 	depends on DEBUG_KERNEL
+	select CONSOLE_POLL if SERIAL_CPM_CONSOLE
 	help
 	  Include in-kernel hooks for the xmon kernel monitor/debugger.
 	  Unless you are intending to debug the kernel, say N here.