[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>

arch/arm/mach-rpc/riscpc.c

@@ -140,7 +140,7 @@ static struct plat_serial8250_port serial_platform_data[] = {
 
 static struct platform_device serial_device = {
 	.name			= "serial8250",
-	.id			= 0,
+	.id			= PLAT8250_DEV_PLATFORM,
 	.dev			= {
 		.platform_data	= serial_platform_data,
 	},