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: c374e00e ("[POWERPC] Add early debug console for CPM serial ports.")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230701054714.30512-1-rdunlap@infradead.org

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