powerpc: allow PPC_EARLY_DEBUG_CPM only when SERIAL_CPM=y

In a randconfig with CONFIG_SERIAL_CPM=m and
CONFIG_PPC_EARLY_DEBUG_CPM=y, there is a build error:
ERROR: modpost: "udbg_putc" [drivers/tty/serial/cpm_uart/cpm_uart.ko] undefined!

Prevent the build error by allowing PPC_EARLY_DEBUG_CPM only when
SERIAL_CPM=y.

Fixes: c374e00 ("[POWERPC] Add early debug console for CPM serial ports.")
Signed-off-by: Randy Dunlap <[email protected]>
Reviewed-by: Pali Rohár <[email protected]>
Reviewed-by: Christophe Leroy <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Link: https://msgid.link/[email protected]

Author: rddunlap

arch/powerpc/Kconfig.debug

@@ -240,7 +240,7 @@ config PPC_EARLY_DEBUG_40x
 
 config PPC_EARLY_DEBUG_CPM
 	bool "Early serial debugging for Freescale CPM-based serial ports"
-	depends on SERIAL_CPM
+	depends on SERIAL_CPM=y
 	help
 	  Select this to enable early debugging for Freescale chips
 	  using a CPM-based serial port.  This assumes that the bootwrapper