Fix spelling of 'platform' in comments and doc

Replace platfrom -> platform.

This is a frequent spelling bug.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/arch/powerpc/kernel/legacy_serial.c b/arch/powerpc/kernel/legacy_serial.c
index 9ddfaef..035ada5 100644
--- a/arch/powerpc/kernel/legacy_serial.c
+++ b/arch/powerpc/kernel/legacy_serial.c
@@ -469,7 +469,7 @@
 		return -ENODEV;
 
 	/*
-	 * Before we register the platfrom serial devices, we need
+	 * Before we register the platform serial devices, we need
 	 * to fixup their interrupts and their IO ports.
 	 */
 	DBG("Fixing serial ports interrupts and IO ports ...\n");