[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/ppc64/kernel/setup.c b/arch/ppc64/kernel/setup.c
index d0bb68a..bfa8791c 100644
--- a/arch/ppc64/kernel/setup.c
+++ b/arch/ppc64/kernel/setup.c
@@ -1283,7 +1283,7 @@
 
 static struct platform_device serial_device = {
 	.name	= "serial8250",
-	.id	= 0,
+	.id	= PLAT8250_DEV_PLATFORM,
 	.dev	= {
 		.platform_data = serial_ports,
 	},