[SERIAL] Use an enum for serial8250 platform device IDs

Rather than hard-coding the platform device IDs, enumerate them.
We don't particularly care about the actual ID we get, just as
long as they're unique.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/arch/ppc/syslib/mpc83xx_devices.c b/arch/ppc/syslib/mpc83xx_devices.c
index 5aaf0e5..95b3b8a 100644
--- a/arch/ppc/syslib/mpc83xx_devices.c
+++ b/arch/ppc/syslib/mpc83xx_devices.c
@@ -165,7 +165,7 @@
 	},
 	[MPC83xx_DUART] = {
 		.name = "serial8250",
-		.id	= 0,
+		.id	= PLAT8250_DEV_PLATFORM,
 		.dev.platform_data = serial_platform_data,
 	},
 	[MPC83xx_SEC2] = {