[PARISC] Make Serial MUX depend on a specific bus type.

Depend on GSC, not PARISC.  Machines without GSC don't have a MUX.
Signed-off-by: Matthew Wilcox <willy@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>

drivers/serial/Kconfig

@@ -507,7 +507,7 @@ config SERIAL_SUNSU_CONSOLE
 
 config SERIAL_MUX
 	tristate "Serial MUX support"
-	depends on PARISC
+	depends on GSC
 	select SERIAL_CORE
 	default y
 	---help---